Cartridges Rims: Rimmed vs. Semi-Rimmed vs. Rimless vs. Belted vs. Rebated - Part III

In our last couple of posts, we looked at a few cartridge rim types: the rimmed cartridge, semi-rimmed and rimless types. We will look at a couple more rim types in today's post: the belted type and the rebated rim type.

As we noted in our previous post, the basic problem with rimmed cartridges was reliable feeding from box magazines, as the cartridge rims would interfere with each other in this type of magazine. One way to solve this was to reduce the diameter of the rim, as we saw with the semi-rimmed type of cartridge. Of course, the smaller rim made it trickier to headspace the cartridge in the chamber properly. Around the same time, another type of cartridge was introduced in 1905 to solve both issues: the belted cartridge.

The belted cartridge design originated in England and was designed by the famous sporting gun manufacturer, Holland & Holland. A belted cartridge is similar to a rimless cartridge in that the rim is around the same diameter as the cartridge case and there is an extractor groove in front of the rim for the extractor claw to fit in and pull out a spent cartridge. The belted cartridge differs in that in front of the extractor groove, there is a raised ring in front of the extractor groove.

The belt acts similar to the rim for the purpose of headspacing the cartridge in the chamber properly. This design allows smooth feeding through box magazines, but also has the advantage of providing positive headspacing, just like a rimmed design. Most belted type cartridges are designed for high-powered hunting rifles.

Headspacing on a belted cartridge. Click on the image to enlarge. Public domain image.

The origin of this type of cartridge had to do with when black powder was being replaced by smokeless powders, specifically cordite. As we saw in the linked article about cordite previously, cordite is composed of long strings of a light brown color, which are packed into a cartridge case in bundles like spaghetti. The prevailing production method of these cartridges in England consisted of inserting small bundles of cordite into a straight-walled case, which was then necked down to the final shape and the bullet was seated. Because of the long strings of cordite, cartridge cases using this propellant tend to have long sloping shoulders.

A .375 Holland & Holland magnum belted cartridge

When these cordite cartridges were first developed, most rifles were still single shot designs, so they were designed as rimmed cartridges. However, as the bolt-action rifles started to become popular, there began a demand for proper feeding from box magazines and hence, the belted cartridge was developed. The first belted cartridge was the .400/375 Holland & Holland Belted Nitro Express cartridge, and it was specifically developed to compete against the German 9.5x57mm Mannlicher-Schonauer cartridge, which was being adopted by Holland & Holland competitor in England, Westley Richards. However, soon after, a German gunmaker named Otto Bock designed the 9.3x62 mm Mauser cartridge. This cartridge was made to be fired out of the Mauser M1898 rifle, which was designed to be mass-produced and cheaper than most British rifles at that time. The cartridge and rifle rapidly became popular with African hunters, because of its all-round capability to be used against animals ranging from the smallest antelopes to the largest elephants. In response to this, Holland & Holland developed the .375 Magnum Belted cartridge in 1912. The belted design allowed cases to feed and extract reliably in the tropical environments found in India and Africa. The .375 H&H Magnum rapidly became one of the most popular all-round hunting cartridges in the world, and in many regions of the world, it is considered to be the legal minimum caliber allowed to be used to hunt large animals.

Interestingly, in the US, the belted cartridge has become synonymous with the word "magnum" and there are several calibers of belted cartridges available, such as: .257 Weatherby Magnum, .300 Weatherby Magnum, .375 Winchester Magnum, .350 Remington Magnum etc.

Rebated cartridge: In this type of cartridge, the rim of the cartridge has a noticeably smaller diameter than the body of the cartridge case. The rim is only used for extraction purposes, and proper headspacing is achieved by using the cartridge mouth or bottleneck body shape. The rationale behind this type of cartridge is to offer increased case capacity (and therefore, more power), without changing the bolt face of the weapon and thereby, keeping most of the other parts of the weapon unchanged.

For instance, in the 1980s it was desired to increase the power of police pistols which use 9x19mm parabellum cartridge. In response to this, Evan Whildin, a vice-president of Action Arms, designed the .41 Action Express cartridge.

A .41 Action Express cartridge on the left, compared to a 9x19mm Parabellum cartridge on the right.

Click on the image to enlarge. Public domain image.

The image above shows a .41 Action Express (.41 AE) cartridge on the left, compared to a 9x19 mm. Parabellum cartridge on the right. The reader will immediately notice that the cartridge on the left is fatter and longer, but what is interesting to note is that the two cartridges have the same sized rims at the bottom. In the case of the .41 AE, since the case body is fatter, the rim is actually smaller diameter than the case body.

The idea behind the .41 AE was that it allows converting a 9 mm. pistol to use this cartridge, merely by replacing the barrel, mainspring and magazine. Since it has the same sized rim as the 9x19 mm., the other parts of the pistol, such as the extractor claw, bolt, firing mechanism etc., can be reused and therefore, it keeps the total cost of converting the weapon relatively low.

However, when it was introduced, many of the ammunition manufacturers backed the .40 S&W cartridge, which had similar performance, and therefore the .41 AE cartridge didn't become popular. Nevertheless, the idea of using a rebated rim cartridge to interchange with another weapon stayed on. For instance, the .50 Action Express (.50 AE) cartridge is designed to be used with the American/Israeli Desert Eagle pistol. The rim of the .50 AE is the same diameter as the .44 Remington Magnum cartridge, which was the most common caliber cartridge used by the Desert Eagle. By interchanging only the barrel and magazine, a Desert Eagle originally designed for .44 magnum, can be used to fire the .50 AE cartridge.

Other cartridges that use a rebated rim design include Winchester Short Magnum, Remington Ultra Magnum, Winchester Super Short Magnum, Remington Short Action Ultra Magnum, the .50 Beowulf etc. The .50 Beowulf has the same sized rim as the 7.62x39mm cartridge used by AK-47 and AKM rifles and is designed to be used by modified AR-15 rifles.

Happy Halloween everyone and stay safe!

Cartridges Rims: Rimmed vs. Semi-Rimmed vs. Rimless vs. Belted vs. Rebated - Part II

In our last post about various cartridge rim types, we looked at the rimmed cartridge. As we noted in the last post, rimmed cartridges are some of the earliest metallic cartridges developed and they work very well with revolvers and shotguns and rifles with tubular magazines, but not so well with box magazines. There are ways to work around this problem by carefully placing the cartridges in the magazine, as was done by the Lee Enfield and Mosin-Nagant rifles, which we saw in the last post, but these rifles have relatively small sized box magazines with less capacity.

Since box magazines can fit more cartridges in a smaller package than tubular magazines or revolvers, by the end of the 19th century, more and more weapons (especially military weapons) began to feature them and with increasing magazine capacities as well. Therefore, the cartridge rim type needed to be fixed to work well with larger capacity box magazines. There were two different cartridge rim types that were developed to fix this issue:

1. Semi-rimmed type
2. Rimless type

Semi-rimmed cartridge: This was one of the first attempts to fix the issue with rimmed cartridges and box magazines. The diameter of the rim is slightly larger than the case diameter, but only marginally so. 

Click on the image to enlarge. Public domain image.

The rim is still used for headspacing the cartridge to the proper depth in the chamber. However, since the diameter of the rim is barely larger than the case, there is less interference with the rim of the next cartridge in the magazine. The rim is wide enough that it is also used by the extractor to pull the cartridge out of the chamber. Examples of semi-rimmed cartridge include the .38 ACP (as illustrated above), the 6.5x50 mm. Arisaka cartridge, the .401 Winchester etc. This type of cartridge is not seen much since the rimless type of cartridge (which we will see below) was developed, but there are still some new cartridges of this type, for instance the .500 S&W Magnum cartridge developed in 2003.

A .500 S&W Magnum cartridge. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported by Peter Gnanapragasam

The .500 S&W Magnum rim is used to headspace the cartridge properly in a revolver, but it can also be used in other magazine types because it is a semi-rimmed cartridge.

As manufacturing technologies for cartridges improved, it became possible to manufacture cartridges accurately enough to headspace off the case mouth (for straight cartridge cases) or case shoulder (for bottlenecked cartridge cases). This led to the development of another cartridge type: the rimless type.

Rimless cartridge: Despite the name, rimless cartridges do have a rim. However, in this type of cartridge, the diameter of the rim is almost the same as that of the cartridge case body. There is only one purpose of the rim: it is used by the extractor to pull the cartridge out of the chamber. There is a groove between the rim and the case body, into which the lip of the extractor can engage to pull the cartridge out,

Since the diameter of the rim is the same as that of the case, obviously we cannot use the rim for headspacing the cartridge in the chamber. So how does it work for these cartridges? The following images show us how this is done:

In the case of straight cartridge cases, the case mouth is not crimped onto the bullet, as is done with revolver cartridges. This leaves a little projection in the case mouth that can be used to headspace the cartridge in the chamber correctly:

Click on the image to enlarge. Public domain image.

The above image shows a .45 ACP cartridge loaded into the chamber. Since the cartridge rim is the same diameter as the case body, this cartridge is headspaced against the throat of the cartridge.

Another way to do it is to shape the cartridge into a bottle-neck shape. For this type, the headspacing happens against the shoulder of the cartridge.

Click on the image to enlarge. Public domain image.

As you can see in the image above, the contact point at the shoulder of the cartridge ensures that the cartridge fits into the chamber at the proper depth.

As manufacturing technologies of cartridges improved in the late 1890s, it became possible to mass produce rimless cartridges that would headspace correctly. The rimless cartridge type quickly became the most popular type of cartridge and has remained so to the current day. Since it has no protrusions to complicate the feeding process, the rimless cartridge type has become well suited for most higher capacity modern magazine types, e.g. box magazines, drum magazines, ammunition belts etc. The easier feeding process also made it possible to produce modern rapid-firing weapons, such as machine guns, sub machine guns etc. Some early rimless cartridges include the 9x19 mm. Parabellum pistol cartridge (Luger cartridge), the .30-03 cartridge from 1903, followed by the .30-06 from 1906, the .45 ACP designed by John Browning in 1904 etc. All these cartridges are still widely used currently, along with more modern designs, such as the 7.62x51 mm. NATO / .308 Winchester and 5.56x45 mm. NATO / .223 Remington cartridges.

In our next post, we will study a couple more types of cartridge rims.

Cartridges Rims: Rimmed vs. Semi-Rimmed vs. Rimless vs. Belted vs. Rebated - Part I

In the next few posts, we will study different types of cartridges by a specific part of the cartridge: the rim. We will discuss five different types of cartridge rim types: Rimmed, Semi-Rimmed, Rimless, Belted and Rebated rims. We will study the features and differences and the reasons that these were manufactured throughout history.

So, first let's start with the definition of a rim. If you look at the back part of any metallic cartridge case (the end opposite the bullet), you will see a sort of a flange at the base of the cartridge. This flange is called a "rim" and has existed ever since the first metallic cartridge was invented. It serves multiple purposes:

• During manufacturing the cartridge, it helps hold the case in position while the propellant and bullet are being loaded into the case.
• It provides a place for the firearm's extractor to latch on to, to pull out a fired cartridge case out of the chamber.
• In some cartridge types, it helps to headspace the cartridge (i.e.) place it into position in the chamber at the correct depth.
• In a particular type of cartridge called rimfire cartridge, the rim contains the priming compound that serves to ignite the propellant of the cartridge, when struck on the rim.

Rims can be added to a cartridge case by various methods: stamping, pressing, casting, molding etc.

With that said, let us look at various rim types. The first one we will study today is the Rimmed cartridge.

This is the oldest type of cartridge and dates back to the time when metallic cartridges were first invented. These cartridges have a rim that is quite a bit larger in diameter than the base diameter of the cartridge. The image below shows a rimmed cartridge.

A rimmed .22 LR cartridge. Click on the image to enlarge. Public domain image

Back in the old days when metallic cartridges were invented, mass manufacturing technologies were not so precise and cartridges of the same caliber would have varying lengths. Therefore, there needed to be some way to hold the cartridge to the proper depth in the chamber so that the firing pin could impact it. Providing a rim larger than the diameter of the cartridge case proved to be a simple solution to this problem, since it could be manufactured cheaply.

Click on the image to enlarge. Public domain image.

As you can see in the above figure, the rim of the cartridge is what prevents it from sliding all the way down through the barrel, since it is significantly larger than the hole in the barrel. Therefore, the rim provides positive headspacing. Since the cartridge headspaces on the rim, the overall length of the cartridge is not critical. Back when cartridge manufacturing technologies were fairly basic, you can see how rims solved the problem of seating the cartridges in the chamber correctly.

The rim also provides a secondary function. Firearms such as shotguns and revolvers need to have some way to easily extract the cartridge cases. The rims provide the means for the extractor to hook on to them and pull out the cartridges from the cylinder (for a revolver) or chamber (for a shotgun):

Most revolvers and shotguns still use rimmed cartridges to this day.

For the .22 LR cartridge (which is the most popular cartridge in the world), the rim also serves a third function. The .22 LR belongs to a family of cartridges called the rimfire cartridges. The patent for rimfire cartridges date back to 1831. The idea is that the priming compound is placed on the entire rim and the rim is designed of thin material. When the rim is struck, it ignites the primer, which burns along the entire rim and ignites the main propellant. Back when black powder was not so high-quality, this provided a reliable source of ignition. The .22 LR is only one of a family of rimfire cartridges, but it is the most popular cartridge in the world and has been in production since 1887.

One more interesting thing about rimmed cartridges is that since they headspace on the rim, it is possible for a firearm that is designed to fire longer cartridges to safely fire shorter cartridges if they have the same sized rim. For instance: 

From left to right: .22 CB, .22 Short and .22 Long Rifle (.22 LR) cartridges

Public domain image.

The above three cartridges are of different lengths, but the cartridge case diameters and the bullet diameters are the same and they all have the same sized rims. This means that a firearm that is designed to fire the longest one of the three (the .22 LR) can also fire the lesser powered cartridges, the .22 CB  and .22 Short. This is because the three cartridges all headspace to the same depth in the chamber because their rims are all the same size.

Similarly, .38 Special cartridges may be fired from a revolver designed for .357 Magnum because the two cases share the same rim diameter (and the .357 revolver is designed to fire higher pressures than what the .38 Special cartridge produces).

A word of warning: While different sized cartridges may fit into chambers designed for other cartridges, it is not always a good idea to try this out. For instance, .38 Long Colt, .38 Special and .357 Magnum cartridges all headspace the same, but firing a .38 Special or a .357 Magnum out of a revolver designed for .38 Long Colt is a bad idea, since the revolver is not built to the pressure that these cartridges can produce.

In the metric system of naming cartridges, a capital "R" added to the end of the cartridge designation indicates that this is a rimmed cartridge. For example, 7.62x54mmR is a cartridge that has a 7.62 mm. diameter bullet and the "R" at the end indicates that this is a rimmed cartridge case. The same is true with the 5.6x35mmR (known in the US as .22 Hornet), 7.7x56mmR (a.k.a .303 British), 9x33mmR (a.k.a. the .357 Magnum) etc.

Rimmed cartridges work very well with revolvers and shotguns, as well as some early repeating rifles that loaded from tubular magazines. Unfortunately, they don't work so well with firearms using box magazines, because the rims tend to interfere with each other during the reload cycle. Since the rims don't ride easily over each other, the rim of the cartridge being chambered often tries to strip the round beneath it in the box magazine. However, certain rifles (notably the British .303 Lee Enfield and the Soviet Mosin-Nagant) solve this problem by carefully arranging the cartridges when the magazine is initially loaded, so that the rim of each case is loaded ahead of the round beneath it. If this arrangement of the cartridges is not done properly, there will be misfeeds and jams with box magazines. We will study how this was solved in the next post, when we study about semi-rimmed and rimless cartridges. 

The MP 18 Submachine Gun

We have studied the evolution of different submachine guns in the last few posts, in reverse order of appearance. Today, we will study the submachine gun that started off this whole class of weapons, the MP 18 submachine gun, otherwise called the MP 18/I or the MP 18.1 or the Bergmann MP 18.1.

During World War I, tactics that were used successfully in previous wars, such as marching lines of infantry and charging cavalry, were rendered useless by artillery and heavy machine guns. With such huge firepower available, masses of infantry soldiers attempting to charge across a battlefield were practically committing suicide. This forced a change in the infantry tactics and soldiers now dug deep trenches across the battlefield and fought from inside them. New tactics were needed to fight in the trenches. For instance, battles inside trenches were fought at very short ranges because trenches were narrow and twisted. Also, since trenches often contained more enemy soldiers defending it than the attackers, it was necessary to clear the trench before the defenders could mount a counter-attack. What was needed was a small caliber weapon with a high rate of fire, but small enough to be used in narrow trenches and light enough to be carried by a single infantry man.

Heavy machine guns had a high rate of fire, but were heavy, required multiple people to move them and needed a good amount of room to operate. Besides, the range of a heavy machine gun did not matter much inside a trench. Pistols and revolvers were small and light, but didn't have a high enough rate of fire or enough ammunition capacity. Rifles were also light compared to heavy machine guns, but were longer (and heavier) than pistols and revolvers and harder to use inside narrow trenches. They also didn't have a high rate of fire or high capacity and used larger cartridges which were unnecessary in narrow trenches.

In 1915, the German authorities attempted to modify existing semi-automatic pistols, the Luger and the Mauser C96, to use larger magazines and fire in automatic mode. However, these efforts were not successful because the pistols were so light that it was hard to aim them when firing in automatic mode. The German Rifle Commission determined that a new class of weapon was required, one that could fire pistol ammunition, but was designed to fire in fully automatic mode from the very beginning.

The design team led by Theodor Bergmann of Bergmann Waffenfabrik started working on a new design to fulfill this requirement. One of the members of the small design team was a talented designer named Hugo Schmeisser. The design they came up with was adopted in the German military in 1918 and was named the Maschinenpistole 18/I or MP 18.1. No one really seems to know what the "I" designation is, but its successor was named the MP 28/II.

The MP 18/I submachine gun. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license by Edmond Huet.

The MP 18 was made using high quality components and designed to use 9x19 mm. Parabellum cartridges, the same as that used by the Luger pistol. The receiver was machined from a thick tube, unlike later submachine gun models which used much thinner tubes. The bolt was also machined from a single block of steel. It was designed as an open bolt blowback weapon, a feature that was copied by practically every submachine gun designed after it, until about 1970 or so. Schmeisser had already designed several blowback pistols for Bergmann Waffenfabrik, so he adopted the same principle for a larger weapon system. Since adding a high capacity magazine to a pistol made it cumbersome and hard to control in automatic firing modes, he designed the new weapon with a traditional-style wooden body, much like a rifle, so that it would be easier to handle. A barrel shroud was added around the barrel, to counter the overheating of the barrel when fired in full automatic mode. The magazine feed was offset to the left of the receiver and the charging handle was located on the right. The weapon was only designed to fire in automatic mode, but since it had a rate of fire of around 500 rounds per minute, it was possible to fire single shots by pulling and releasing the trigger rapidly.

Interestingly, the early design for the MP 18 used a 20-round box magazine, but the German military insisted that the new weapon use a 32-round "snail" drum magazine (the TM 08 magazine), which was originally designed for the Luger pistol.

A TM 08 snail drum magazine. Click on the image to enlarge.

Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported license by Edmond Huet.

The snail drum magazine is not a true drum magazine, but is essentially a box magazine folded into a spiral shape. The cartridges are arranged inside it in a spiral pattern and a special loading tool is required to load it. The magazines used with the MP 18 needed to have a special sleeve to prevent the magazine from being inserted too far into the weapon. This snail drum magazine was heavy, awkward and hard to load and was one of the weaknesses of the MP 18.

Nevertheless, the MP 18 was adopted by the German army in early 1918 and about 5,000 (or 10,000) were manufactured. Even though they were not used for very long, these weapons proved to be very useful in trench warfare. In fact, when World War I ended, the treaty of Versailles explicitly forbade Germany from manufacturing any submachine guns. The 32-round snail drum magazine was also prohibited from being manufactured by the same treaty. However, Bergmann Waffenfabrik continued to manufacture this weapon in secret until about 1920 and a total of about 35,000 weapons were made. After that, Bergmann sold the design and manufacturing license to SIG of Switzerland, who started to sell it as the SIG Bergmann 1920. The Swiss produced different versions that could fire 9x19 mm. cartridges (the same as the Luger) and the 7.63x25 mm. cartridges (the same as the Mauser C96). It continued to be used by police forces in the Weimar republic, as well as China, France, Finland etc. Hugo Schmeisser modified the design to use the 20 round box magazine that he'd originally designed for it, and later, 40 and 50 round box magazines were also made for it. In 1928, he modified the design to have a selector switch and this new model was called the MP 28/II.

The MP 18 was the world's first submachine gun (technically, the Italian Villar-Perosa from 1915 was also an automatic weapon firing pistol caliber ammunition, but it was originally designed to be used as a mounted weapon in aircraft). It was influential in the designs of submachine guns that followed it. For instance, it was the MP 18 that inspired the Finns to invent their own Suomi KP/-31 submachine gun that we saw in our previous post. The British designed their Lanchester submachine gun based on the MP 28 and even made it use the same magazines as the MP 18 and MP 28. The later Sten gun could also use the MP 18 box magazines. Practically every submachine gun designed after it until about 1970, used a blowback system of operation and fired from an open bolt.

While the MP 18 was an influential design, it was also heavy and somewhat expensive to produce because key parts were machined from solid steel blocks. Later submachine guns were designed to be manufactured much more quickly and at lower costs, by using stamping and spot welding techniques.

Incidentally, it is ironic that Hugo Schmeisser's name is not associated much with the MP 18, a weapon where he made very significant contributions to the design. However, his name is popularly associated with the MP 40, a weapon that he made very little contribution to. It turned out that the German War Office mandated that the MP 40 use a magazine design that he'd actually patented for the MP 18 and MP 28 and therefore, his name somehow got associated with the MP 40 and it is referred to as the "Schmeisser submachine gun" in many countries.

The Suomi KP/-31 Submachine Gun

In our last post, we had studied about the Soviet PPSh-41 submachine gun. In today's post, we will study one of the main inspirations for the PPSh-41, the Finnish-made Suomi KP/-31.

During World War I, the German designer, Hugo Schmeisser had developed the MP-18 submachine gun, which his employer, the German manufacturer, Theodor Bergmann, started to produce in his factory by 1918 and about 10,000 were made before the war ended soon after. This weapon was the first practical submachine gun used in combat (though not the world's first submachine gun). Even though this weapon was only used for a short period of time, it gained such a reputation that the treaty of Versailles explicitly specified submachine guns among the list of weapons banned from the German military. One of the weaknesses of the MP-18 was its snail-drum magazine, so shortly after World War I, Hugo Schmeisser improved the design to use box magazines. Bergmann continued to make this weapon in secret for a few years after World War I ended and about 25,000 (or 35,000 depending on who you ask) were made in total. However, since Bergmann could no longer manufacture these weapons openly, they finally sold the improved design to the Swiss company SIG (Schweitzerische Industrie-Gesellschaft) in 1920, which started to sell the improved design as the Bergmann Submachine Gun. SIG sold Bergmann models of various calibers to several countries, such as China, Japan and Finland, between 1920 and 1927, when they finally stopped making these weapons.

The Finnish army wasn't interested in this weapon initially, but the Finnish Civil Guard (Suojeluskunta) were interested and bought about 1000 Bergmann submachine guns from SIG in the early 1920s. Around this time, a talented Finnish weapon's designer named Aimo Lahti thought the Bergmann was too expensive and the reliability could be improved as well. He made his first prototype in 1921, a model that fired the .32 ACP cartridge and was about 11.8 inches (30 cm.) long and it was handmade by a blacksmith in Viiala. By 1922, he had a real working prototype (M/22) which was built by a factory with machine tools (Leskinen & Kari in Tampere). He tried to get the Finnish military interested in this weapon, but they refused because it wasn't really production-ready yet, but some members of the Keski-Suomi regiment did express interest in it. Therefore, in 1924, Lahti established Konepistooli Oy (Translation: Machine Pistol Corp.) with members of this regiment, Captain V. Korpela, Lt. Y. Koskinen and Lt. L. Boyer-Spoof. (Korpela had to leave the company later, as he was trying to sell the submachine gun to other countries without permission of the other shareholders). 

Lahti continued to make improvements and by 1924, the Finnish Defence Ministry got interested and bought about 100 submachine guns. At this time, Konepistooli Oy needed every sale they could get. By 1926, the M/26 model was introduced, which used a very unique 36-round magazine, that was not used in any weapon before then.

The Suomi M/26 submachine gun. Click on the image to enlarge.

Note the unique curved magazine shape of this weapon. The M/26 cost about half the price of the Bergmann, but it did not sell well. It also had some feeding issues and shared some of the weaknesses of the original MP-18 and Bergmann. The stock was also not strong enough for military use. Lahti succeeded in fixing these issues in his new model, the M/31.

One of the improvements in the M/31 was to remove the excess room in front of the bolt (which sometimes caused cartridges to turn sideways during loading) to prevent jamming problems. A quick release mechanism allowed the barrel to be rapidly replaced. An improved muzzle brake reduced the muzzle climb during shooting in automatic mode. The most important change was the cartridge used, as the M/31 was designed to use the 9x19 mm. parabellum cartridge that many other countries were using at that time. Because of the 9x19 mm. cartridge, two new magazines, a 20 round box magazine and a 40 round drum magazine were developed as well.

The Finnish Defence Ministry were very interested in the M/31, but Konepistooli Oy lacked the facilities for mass production. Luckily, an engineer named Oscar Ostman was a personal friend of Aimo Lahti. Ostman was the CEO of a Finnish company called Tikkakoski Rauta ja Puuteollisuusyhtio (Translation: Tikkakoski Iron and Wood Products Ltd.), which originally was a metal workshop, but also had experience in making firearm parts (like barrels for rifles and machine guns) for the Finnish military. Interestingly, the major shareholder of Tikkakoski was a German weapons dealer named Willi Daugs. Tikkakoski bought the rights to produce the M/31 from Konepistooli and called it the Suomi KP/-31 (KP standing for Konepistooli (i.e. Machine Pistol) and 31, since it was manufactured in 1931). 

The Suomi KP/-31 submachine gun. Click on the image to enlarge.

Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported license by Mbeesb

The 20 round box magazine could actually be filled with upto 25 cartridges, but it wasn't too reliable when it was filled with 25 cartridges and it was felt that 20 rounds was too little capacity for the magazine and so, the manufacture of these stopped by 1939. The 40 round drum magazine also had some reliability issues, but its bigger problem was loading it -- cartridges had to be inserted into the drum with the cartridges standing on the tips of the bullets. The slightest vibration could cause the cartridges to fall on their sides inside the magazine and the user would have to dump all the cartridges out and start from the beginning again. Luckily, Lt. Y. Koskinen, who was one of the other founders of Konepistooli Oy, came up with the design of an improved 71 round drum magazine, without Aimo Lahti's finding out until it was ready for production. This 71-round magazine was the most well known and successful magazine design of the KP/-31. In fact, the Soviets were so impressed by it, that they cloned it for their PPSh-41, as we saw in the previous article. A 50 round quadruple column casket box magazine of Swedish design was also made for the KP/-31, but this magazine proved vulnerable to small dents and therefore, it was mostly issued with the 71 round drum magazine.

When the Winter War started, Finland only had about 4000 of these in service, but after the Winter War ended and as the Continuation war started, production had ramped up to about 1400-1500 weapons made per month. Finnish soldiers used this weapon with deadly effect against the Soviets. This weapon showed what a useful weapon the submachine gun was to modern armies and other military forces were quick to adopt this concept as well.

The KP/-31 had a number of interesting features about it. It was a blowback action using Advanced Primer Ignition. This means the firing pin ignites the primer of the cartridge, before the bolt stops into battery. The bolt's momentum keeps the cartridge case locked in the chamber until the peak chamber pressure from the gases has dropped to a safe level. Unlike later submachine gun designs, this was largely built by machining the components and therefore took longer to manufacture. For instance, the receiver was machined from a solid forged steel block. The steel used for the barrel and receiver was the best Swedish made chromium-nickel steel that was available during that time. Some of the original barrels for the pre-1931 models were made by Birmingham Small Arms (BSA) in England, but Tikkakoski started making their own barrels by 1931, and many were also supplied by the precision competition rifle manufacturer, Joonas Matarainen. Interestingly, for a submachine gun, it had a quick-detachable barrel and a barrel jacket, features that are usually only found on heavy machine guns. Also, unusually for a submachine gun, each KP/-31 was supplied with two barrels (primary and spare barrel). Each pair of barrels were machined to a very high degree of precision, so that they both had very similar dispersion and point of impact. This allowed the user to quickly change barrels in the middle of a firefight without needing to adjust the sights afterwards! Even though the weapon had a high rate of fire (around 900 rounds per minute), it was surprisingly well-balanced and controllable. 

How accurate was it, the reader asks? Factory testers would shoot 10 shots at a target in semi-automatic mode, using a simple bench rest and all 10 shots would have to hit the bullseye of the target at 100 meters distance. The following two images show the results of tests conducted by the Finnish Army during official acceptance tests.

Click on the image to enlarge.

The diameter of the center circle (the ten circle) of both targets has a diameter of 4 inches (25 mm.). The shots were made from a distance of 100 meters using a simple bench rest. The left image shows 15 rounds fired at this distance using semi-automatic mode. The right image is using a 50 round magazine fired in a single long burst at the same distance, using full-automatic mode. As you can see, the test in semi-automatic mode has a 1 MOA accuracy (and it wasn't uncommon to get sub MOA groups either). In the right image, it can be seen that 48 out of the 50 shots are within the smallest or second smallest circle.

Unlike most other submachine guns, the sights were adjustable to 500 meters distance, which was a bit optimistic, but the weapon was easily capable of hitting targets at 300 meters distance. It was much more accurate than every other submachine gun of that period (and probably even now). In the dense Finnish forests, the range of the KP/-31 was more than adequate and the large drum magazine and fast rate of fire meant it could spray more lead than practically any other automatic weapon of that time. 

One more interesting feature was the cocking handle (charging handle). It was a non-reciprocating design (i.e.) it does not move back and forth when the gun is fired (similar to that of a bolt-action rifle). The user only pulled it back once after loading a new magazine into the weapon, to cock it initially, and after that the charging handle would not move at all. Because of this feature, it didn't have a bolt handle slot and therefore, was less likely to let mud and snow enter into the firing mechanism, which led to greater reliability.

A couple of variants were made to be used inside bunkers and tanks. These had thinner barrel shrouds and no shoulder stock, using a pistol grip instead. These were designed to be used in enclosed spaces where it was necessary to fire through narrow slits. On these variants, the sights located on the left side of the weapon, to make it easier to aim through a slit. The tank version was designed with a special barrel shroud that was permanently attached to the firing port of a 6-ton Vickers tank (the T-26E model). If it became necessary for the crew to abandon the tank, the weapon could be easily removed from the firing port and normal barrel shroud attached to it to use as a regular submachine gun.

One more variant was from 1942. This variant had an "improvement" to add a muzzle brake. This model was called the KP/-31 SJR (SJR from the word "suujarru", the Finnish word for "muzzle brake" or "compensator"). This increased the length of the weapon by 55 mm. Aimo Lahti didn't like the new compensator because the oblique front end of the barrel jacket was found to be as efficient as the compensator. On the other hand, the compensator caused trouble in cold frosty weather, as powder and primer residue would get trapped in the chamber and the resulting sticky mass of condensed water, carbon and salts would stick to the breech bolt and cause misfires. On top of that, the compensator reduced the muzzle velocity a bit as well. This "improvement" enraged Aimo Lahti so much that he attempted to find out who was responsible for this change and have him charged in a military court, but to this day, the identity of the genius responsible remains a classified secret!

The Finns used this weapon against Soviet forces with great success and showed the world how submachine guns could be used in warfare by regular troops and not just as defensive weapons by truck drivers and tank crews. 

The Suomi KP/-31 had superior firepower, excellent accuracy and very high reliability. On the downsides were its weight, high production cost and slower production rate. Finnish infantry soldiers liked this weapon a lot. It was also manufactured under license in Sweden, Denmark and Switzerland and also used by other countries such as Norway, Israel, Germany etc. In 1938, the British Army conducted tests on many different submachine gun models and their Ordnance Board concluded that the KP/-31 was the best weapon by far. However, since the Finns were battling the Soviets at that time, the British concluded that they might not be available for purchase and opted for the Thompson submachine gun instead.

Incidentally, Norway used them until the 1980s and the Vatican Swiss Guard used them until the 1970s (as the Swiss made version of the KP/-31, the Hispano Suiza MP43/MP44). The tank version of the KP-31 (which was described 4 paragraphs above) was still in service in Finland through the 1980s, despite the fact that the T-26E tank that it was designed for, was decommissioned by Finland in 1959!

The PPSh-41 Submachine Gun

In our last post, we studied the German MP40 submachine gun, an influential design. In today's post, we will study a submachine gun of Soviet origin, the PPSh-41.

The PPSh-41 derives its name from the Russian name Пистолет-пулемёт Шпагина which is pronounced as Pistolet-Pulemyot Shpagina (i.e. "Shpagin Machine Pistol"). Shpagin refers to the designer of this weapon, Georgy Semyonovich Shpagin. The number -41 refers to the year that this weapon was first produced.

A PPSh-41 with a drum magazine. Click on the image to enlarge.

Image released under a Creative Commons Attribution-Share Alike 3.0 Unported license by Lposka

The gun was called the "Peh-Peh-Sha" (after the Russian pronunciation of the first letter of each word), or sometimes as "Papasha" (the Russian word for "Daddy"). During the Korean war, US and UK forces who encountered this in the hands of North Koreans and Chinese troops, nicknamed this the "burp gun" on account of the "brrrap" sound it produces when firing in automatic mode.

The origins of this weapon have to do with two reasons. First, when the Soviets fought against Finland in the Winter War of 1939-40, they found that Finnish forces armed with Suomi KP-31 submachine guns could take on superior numbers of Soviet forces fighting with bolt-action Mosin-Nagant rifles at close quarters. The Soviets already had a submachine gun of their own, the PPD-34, but it was hard to produce economically and quickly. Many of the metal parts of the PPD-34 were produced by milling and machining operations, which made it take a long time to produce.

Then, in the summer of June 1941, the Germans launched Operation Barbarossa and invaded the Soviet Union. A large number of Soviet small arms as well as a number of the factories capable of manufacturing small arms, fell into German hands. The Soviets needed a weapon that could be produced quickly and without the use of sophisticated manufacturing technologies.

Enter Georgy Shpagin, a son of a Russian peasant, who had started off life as a carpenter, before being drafted into the Russian army and becoming a gunsmith and later a weapon designer. He took the PPD-40 (an improved version of the PPD-34) as the starting point of his design. While the PPD-40 used mostly machined parts, his new PPSh-41 design largely relied on stamped steel parts. Most of these parts could be manufactured by unskilled workers using equipment that is commonly seen in small workshops and automobile repair garages. Parts were joined together by welding. The PPSh-41 design used less parts than the PPD-40 and could be manufactured in about half the time it took to make a PPD-40. Only the barrel needed a little more sophisticated factory equipment to manufacture and even this was simplified, as will be discussed shortly.

The cartridge used for his weapon design was the 7.62x25 mm. Tokarev pistol cartridge, which was already in service with Soviet forces for some years previously. To simplify the production of barrels, he often used existing barrels designed for the Mosin-Nagant M1891 rifle, which were already bored for 7.62 mm. A single long M1891 barrel would be cut into two shorter barrels for the PPSh-41 as they both use 7.62 mm. diameter cartridges, and then each shorter barrel would have a chamber machined to accept the 7.62x25 mm. Tokarev cartridge. The barrels were chrome lined for extra reliability, but most of the other parts were much more cheaply produced.

The weapon itself fires from an open bolt using the blowback action (similar to all the designs we studied in the last few posts). Earlier models had a selector lever to select the firing mode and later models were designed to fire in full automatic mode only. The firing rate of this weapon is about 1000 rounds per minute, which is the highest of all the submachine guns during World War II. The receiver is hinged to make disassembly and cleaning the weapon in the field easy to do. The chrome lining in the barrel enables it to use corrosive ammunition and not be cleaned for a while in the field.

The first prototypes went into production in November 1941 and by the spring of 1942, the factories were making them at an incredible rate of 3000 per day and by the end of 1942, over 1.5 million of these were made.

Initially, the PPSh-41 was designed to use a drum magazine of 71 round capacity, which was an exact copy of the Finnish Suomi KP-31 submachine gun that the Soviets had encountered in the Winter War. The drum had a spring loaded feed mechanism that the user would have to wind up like a watch to use. However, the drum magazine had reliability issues if it was loaded to its full capacity of 71 rounds and therefore, users would often load less than 64 or 65 rounds to avoid this problem. The drum magazine was also made of a thin sheet metal of 0.5 mm. thickness. This meant that it was pretty easy to bend, which could also cause feed failures. Also, the springs inside the drum magazines would often weaken after a while and make feeding unreliable. On top of that, it took a long time to load a drum magazine. Therefore, newer versions of the PPSh-41 were supplied with 35 round box magazine instead, after about 1942.

A PPSh-41 with a box magazine. Click on the image to enlarge.

The box magazines were easy to carry around in pouches and more reliable than the drum magazines. The thickness of the metal used to make magazines (both box and drum magazines) was later changed from 0.5 mm to 1 mm., which made them much harder to bend accidentally. Many infantrymen would carry one drum magazine in the gun and several box magazines in pouches.

The Soviets made about 6 million of these weapons by the end of World War II. Some of these were later gifted to other communist countries and some African countries. The design of the PPSh-41 was also shared with some countries, most notably, the North Koreans and the Chinese, who made their own versions of this weapon during the Korean war. The Chinese also gave their design to the Vietnamese, who made their own version during the Vietnam war. US and UN troops who faced this weapon in Korea and Vietnam nicknamed this the "burp gun" on account of the noise it made when firing. The weapon still remains in use by some irregular forces around the world.