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 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.

What the heck is an "Arshin"?

Imagine you are now the proud owner of a classic Russian Mosin-Nagant M-91 rifle and are excited to try it out for the first time. So you take it out to the range, adjust the iron sights for a target 400 meters away and shoot at it. Upon shooting a few times, you examine the target and notice that you're not hitting where you're aiming at. Is there something wrong with the rifle? Actually, the answer may have to do with your misunderstanding about how the sight works.

A Konovalov type Mosin-Nagant adjustable iron sight.

In the above image, we see a Mosin-Nagant rifle sight called the Konovalov type. This sight acts as both a  tangent sight (for shorter ranges, marked as 4-12) and a ladder sight (for longer ranges, marked as 13-32). Note that the sight is sort of curved, as seen in the first image. Older Mosin Nagants have a flat shaped sight using the same idea.

The mistake that some people make is assume that these settings are in meters (e.g. 4 = 400 meters, 6 = 600 meters etc.) For Mosin-Nagants manufactured before 1930, this is not true -- in fact, these are calibrated in a unit called "Arshin" (plural: "Arshins" in English, "Arshiny" in Russian). So what the heck is an "Arshin" then?

To answer this question, we must go back to 16th century Russia, where this unit of length first originated. This unit was the Russian equivalent of the English "yard" measurement. Its actual length varied over the years, until Peter the Great standardized it in the 18th century to be about 27.95 inches long (or about 71.1 cm. or 0.78 yards). This continued to be how distances were measured in Russia until some time in 1925, when the Soviet Union officially adopted the metric system throughout the country. Konovalov type sights calibrated in arshins continued to be manufactured for some years after, until 1929.

Therefore, with older Mosin-Nagant rifles built before 1930, the sights were actually calibrated in arshins. Hence, when the slider is pushed to 4, the sights are set to aim at a distance of 400 arshins, not 400 meters. 400 arshins is approximately 285 meters, which explains why the rifle might not be shooting where the user expects it to!

Edit: The editor is indebted to Mr. Bernard Samartsev for his comments and corrections to the original article noted below.