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. 

Firing Different Sized Cartridges?

After all that stuff we read about headspacing, in the last couple of posts, let's look at a tangential topic here. In the image below, we have three kinds of cartridges, all of .22 caliber, but of different lengths:

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

Public domain image.

The three cartridges in question are (from left to right), .22 CB, .22 Short and .22 Long Rifle (popularly known as .22 LR). Now imagine that you have a firearm that is designed to shoot .22 LR. Will it be able to shoot the other cartridges?

To answer this question, we must first note that these are rimmed cartridges, i.e. the rims of the cartridges are wider than the body of the cartridges. Also note that although the overall lengths of the cartridges are different, the diameter of the cartridge cases and bullets are identical. Also, the sizes of the rims in all three cases are identical.

Now, when we first started talking about headspacing two days ago, recall how the headspace for rimmed cartridges is measured.

Click on image to enlarge.

That's right, for rimmed cartridges, the headspace is measured as the width of the rim. Therefore, a firearm that can fit a .22 LR cartridge (the longest of the three) can also comfortably fit a .22 short, a .22 Long and a .22 CB. Since all these cartridges have the same rim diameter and rim width, they also headspace correctly. This means that a .22 LR firearm can fire these without the fear of case separation occurring.

Of course, a few caveats must be noted here. The first is that of the .22 Short, .22 Long and .22 LR, they were all originally designed to use the same propellant, black powder, with the .22 LR having the most quantity in the cartridge (the .22 CB is even more low powered than these three). Therefore, a firearm designed for .22 LR is strong enough to handle the pressures generated by the other cartridges. Also, since the .22 LR is the longest of the four cartridges, the other three can fit in the chamber. Since all four of these are rimfire cartridges, the same firing mechanism can be used for all of them. However, it must be noted that just because a firearm designed for .22 LR can be fired with any of these 4 cartridges does not mean it can function after that. For instance, cartridges may have to be manually fed in one at a time, because a .22 LR magazine may not feed the other cartridges properly due to the different lengths of the cartridges. Also, the recoil forces generated by the smaller cartridges may not be enough to cycle the action of a firearm designed for .22 LR properly. Therefore, depending on the type of firearm and its action, the user may have to manually extract and feed smaller cartridges. The sights will also have to be adjusted, since the smaller cartridges have less power and shoot at lower velocities. Hence, while it is possible to safely fire .22 CB, .22 Short, .22 Long and .22 LR out of a firearm designed for .22 LR, there are many reasons why people do not always do so.

One more major caveat -- there are other so-called cartridges in .22 caliber, for instance, the .22 Magnum (a.k.a. .22 WMR or .22 Winchester Magnum Rifle). This cartridge cannot and should not be fired from a rifle designed for .22 LR. For one thing, the two cartridges are actually of different diameters and lengths. On top of that, .22 WMR uses much stronger propellant that .22 LR and therefore generates higher pressures (which is the reason why they increased the diameter and length of the .22 WMR cartridge, so it cannot fit into a .22 LR firearm chamber, otherwise it could be a dangerous situation because the .22 LR firearm may not be able to handle the higher pressure). A .22 LR cartridge can fit in the chamber of a firearm designed for .22 WMR, but since the cartridge is of smaller diameter, when the cartridge is fired, it may split in the chamber and stick to it.

Therefore, it is always a very good idea to always know the exact ammunition type of a firearm.

Headspacing - II

In our last post, we looked into the subject of "headspacing", we will continue to study our subject in this post. As was suggested in our last post, headspace is the distance between the bolt face and a point in the chamber that prevents further forward movement of a cartridge. Depending on the firearm and the shape of the cartridge, this distance can be different for different types, as the figure below shows:

Click on image to enlarge.

Image is licensed under the Creative Commons Attribution-Share Alike 3.0 Unported by user Amendola90 at wikipedia.org

Now we will look into what happens if the headspacing is not correct. There are two scenarios we must consider here. The first is excessive headspacing, where there is extra space between the bolt face and the cartridge in the chamber. The second is insufficient headspacing, where there is too little space between the bolt and the cartridge.

What if there is excessive headspacing? In this scenario, when the firing pin hits the cartridge, it will move forward into the chamber before detonating. When the propellant explodes, the walls of the cartridge will expand due to heat and firmly stick to the walls of the chamber, preventing rearward motion of the cartridge. However, the thicker base of the cartridge will move backwards, because there is now a gap between it and the bolt face and this will cause the walls of the cartridge to stretch. If the stretching is too much, the walls of the cartridge could rupture and release hot gases into the action and also potentially spray case fragments out from the action of the firearm, which could be hazardous to the shooter or people standing next to him.

Well, what if there is insufficient headspacing in the chamber? In this scenario, the back of the cartridge will stick out of it and the user will not be able to close the bolt fully on the loaded cartridge. The user will not be able to properly operate the firearm if this happens. If the user were to force the bolt to close on the cartridge, this pushes the bullet tightly into the case neck and if the firearm is fired, this will cause excessive pressure to build up inside it, leading to hot gases coming out of the cartridge's primer pocket with similar results as excessive spacing. In worst case scenarios, the excess pressure could cause the action to rupture and cause damage to the gun and its user.

So, how do we determine that a firearm has proper headspacing? We can do that by using a set of gauges, called headspacing gauges. These are measuring instruments that are precisely machined to the SAAMI or CIP or military standards of a cartridge caliber. Typically, these are made of heat-treated steel and are machined to tolerances below 0.001 inches or so. They are made for various calibers by several reputable commercial companies at fairly reasonable prices. Typically, there is a "Go" gauge, a "No-go" gauge and (for military specification rifles), a "Field" gauge.

Headspacing gauges. Top is a "No-go" gauge and bottom is a "Go" gauge

The "Go" Gauge: A firearm must be able to close the bolt with no resistance at all, when a "Go" gauge is inserted into the chamber. This signifies that the firearm is able to meet the minimum length specification for the cartridge. If the bolt does not close on the "Go" gauge, this means that the firearm has insufficient headspacing and may be dangerous to fire, even if the user manages to force standard cartridges to fit in the chamber. Sometimes, the cause of this may be a dirty chamber or bolt face, because the accumulated dirt may be thick enough to prevent the bolt from closing on the gauge. However, if the firearm is clean and the bolt still does not close on the "Go" gauge, it must be taken to a competent gunsmith for adjustments.

If a firearm successfully closes on a "Go" gauge, it means that the firearm at least has sufficient headspace, but it may still have excessive headspace. That can be determined with our next gauge.

The "No-go" Gauge: A new (or overhauled) firearm must not be able to close on a "No-go" gauge. If the bolt closes successfully on a "No-go" gauge, this means the firearm has excessive headspace and there is a risk of cartridge cases rupturing inside the chamber. If the firearm is new or recently repaired, that means it should be returned to the manufacturer immediately.

A used firearm may be able to close on a "No-go" gauge, due to wear of the bolt and chamber surfaces, but this means that it should probably go to a gunsmith for repair soon and it may be possible to fire new factory ammunition in it until then, but reloaded ammunition is probably a bad idea and the firearm may malfunction on slightly out-of-spec cartridges. Here's where the test with the third gauge comes in (especially for firearms built to military specifications).

The "Field" Gauge: The bolt of any firearm, whether old or new, should not be able to close on a "Field" gauge. A bolt that closes on "no-go", but not on a "field" gauge, may be considered close to being unsafe, but may work on new cartridges (and should be sent to a gunsmith soon for possible re-headspacing). However, if the bolt closes on a "field" gauge as well, then it is definitely not safe to fire and should be sent for repairs immediately (or mounted on a wall as a decorative piece!).

Some calibers have a fourth gauge called a "Field II" gauge, for which the bolt should never lock on. This type of gauge is only used by some rifles (for example, Colt uses it to reject M-16 rifles).

It must be noted that gauges are usually manufactured to either SAAMI, CIP or military standards and therefore may have different dimensions, even for the same caliber cartridge. Therefore, it may be possible that a rifle manufactured to NATO specifications, may lock on a "No-go" gauge built to SAAMI specifications, but correctly not lock on a "No-go" gauge built to NATO specifications. This is because military weapons are generally designed to operate with wider tolerances and military ammunition cases are generally thicker than commercial ammunition and can tolerate more stretching without rupturing. Therefore, a military firearm may fail the test using SAAMI gauges, but still be deemed somewhat safe to fire per the military specification gauges. However, if it passes even with using SAAMI gauges, that means it is very likely to work correctly.

Go, No-go and Field gauges are available from many firearm supply stores and cost about $25 to $50 per gauge or about $70 to $140 per set.

Headspacing - I

It has been a while since the last article. Today, we will study an important concept called "Headspacing". This will be a two-part post.

The term "headspacing" is defined as the distance between the face of the bolt and a point in the chamber that prevents further forward movement of a cartridge.

Click on image to enlarge. Public domain image.

In the above pictures, we have a .45 ACP cartridge loaded into a chamber. The cartridge case is a bit larger than the bullet and the chamber is shaped so that it fits the case correctly. Of course, depending on the shape of the cartridge, the headspacing could be different for different firearms.

For instance, some of the earliest metallic cartridges were rimmed. This means, they have a rim at the base of the cartridge that is larger than the diameter of the cartridge case. When this type of cartridge is pushed into a firearm's chamber, the rim positions the cartridge correctly and prevents it from slipping too far into the chamber. Therefore, for a rimmed cartridge, the proper headspacing should be the width of the rim.

Headspacing of a rimmed cartridge. Click on image to enlarge.

The most popular cartridge today, the .22 Long Rifle rimfire cartridge is an example of a rimmed cartridge.

However, it must be noted that most modern cartridges are rimless (i.e.) the diameter of the rim is the same or smaller than the diameter of the case body. Modern cartridges have other ways to ensure that the bullet seats in the chamber correctly. For example, for most pistol cartridges, the case diameter is slightly larger than the bullet and the chamber has a shoulder that the case mouth rests against.

Headspacing on a .45 ACP rimless cartridge. Click on image to enlarge.

In the image above, we see a .45 ACP rimless cartridge seated in a chamber. The case mouth is larger than the bullet diameter and rests against the front of the chamber, thereby correctly seating the cartridge.

Other rimless cartridges, such as bottle-necked cartridges (which are mostly used by rifles), the shape of the firearm's chamber is correspondingly tapered to achieve proper headspacing.

Headspacing on a bottlenecked rimless cartridge. Click on image to enlarge.

In the above image, we see that the chamber is tapered to accommodate the cartridge and the headspacing is the distance from the bolt face to the tapered section.

In the case of belted cartridges (such as the hunting cartridge, Holland & Holland .300 Magnum), the chamber is shaped to seat the forward face of the belt.

Headspacing on a belted cartridge. Click on image to enlarge.

When the H&H .300 Magnum was first being developed, it was difficult to provide proper headspacing for a cartridge with a shallow shoulder design. The solution was to add a belt around the cartridge body. This is similar in function to the rim of a rimmed cartridge, but gave a long enough surface for the cartridges to fit side-by-side in a magazine, without risk of interference during the feed stroke. This idea was later copied by other magnum rifle cartridges as well.

We will study the importance of proper headspacing in the next post.