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After seeing that recent pistol (https://en.wikipedia.org/wiki/FN_Five-seven) has bullet diameter quite similar to one used in modern assault rifle, I started to wonder whether it is possible (and reasonable), to design:

  • pistol and assault rifle of the same bullet diameter, but different ammo length

AND

  • both pistol and rifle are of reasonable performance

AND

  • just in case of running out of rifle ammo, one can load inferior and shorter pistol ammo in to rifle. (without caring any special converting gear)

Would such bullet compatibility be possible without any serious trade offs? (I'm not asking about using the exactly same bullet, but ammo of DIFFERENT length)

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    $\begingroup$ Why not go for one light rifle round? Like the aformentioned 5.7? What are your rifles used for? What must their effective range be? $\endgroup$ – Mormacil Apr 8 '17 at 16:22
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    $\begingroup$ What do you think you're solving by this? What's your use case? Note that it was common to use .357-magnum pistols and .357-magnum lever-action rifles so that you only had to carry one type of ammo... $\endgroup$ – iAdjunct Apr 8 '17 at 21:00
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    $\begingroup$ Muskets allow for this. $\endgroup$ – iAdjunct Apr 8 '17 at 21:02
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    $\begingroup$ "Assault rifle" is just a classification. Wouldn't your gun be better classified as a "submachine gun"? $\endgroup$ – vsz Apr 9 '17 at 11:05
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    $\begingroup$ This discussion of the Schofield Revolver discusses putting the shorter .45 Schofield into revolvers that normally took a .45 Colt. They're both straight sided, rimmed .45 caliber pistol cartridges, but the rim on the .45 Schofield was larger, so you could only put one in every other chamber on a Colt, 3 rounds in a 6 gun. Later they cut down the rim on the .45 Schofield to solve the problem. We're talking a difference of about 4 mm. $\endgroup$ – Schwern Apr 22 '17 at 19:04
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Same width projectile

pistol and assault rifle of the same bullet diameter, but different ammo length

Absolutely. An M-16 is a .22 caliber, as are several pistols.

Note that this isn't what you actually want. There is no indication that any existing pistol can actually fire an M-16 bullet or that any existing pistol bullet would go into an M-16.

Different length ammunition in the same gun

See this for example:

.22 Short

The .22 short was originally developed as a self-defense round in the 1800s, and while you'll see it primarily used in smaller handguns today, you can still find rifles capable of shooting both short and long .22 rounds. However, it is important to note that .22 short rounds fired out of rifles tend to be less accurate than .22 long rounds. 22 short rounds are always rimfire, and contain relatively light powder loads vs. other .22 ammo. If you're planning on stocking up on .22 short rounds, best do it for your .22 handgun, and leave your rifle round purchases to the more reliable .22 long rifle.

The .22 short mentioned here is suggested as being mostly for pistols but usable in some rifles, albeit with lower accuracy than ammunition better designed for that rifle.

Note that this is purely single shot rifles that are bolt action.

Interchangeable ammunition

There are several examples of rifles and pistols that can share ammunition at this page, e.g.:

Rock River Arms LAR-9 CAR A4: This handy, reliable AR is user-friendly and tons of fun to shoot. It comes with a 16 inch Chrome Moly barrel with a 1:10 twist rate, optional R4 or RRA Quad Rail. The six-position tactical CAR stock from RRA is easy to use and locks securely in place, and the gun comes with an A2 flash hider with ½-36 threads. Hogue grips offer a secure, comfortable hold, and the excellent single-stage trigger makes this one of the most accurate 9mm ARs available today. The flattop forged upper offers plenty of space for mounting any optic you’d like, and at just over seven pounds, this is a very comfortable rifle to shoot, even for extended periods at the range. RRA offers a wide variety of options and upgrades, and the CAR A4 is a very good defensive firearm. It’s also loads of fun to shoot. MSRP: $1,180.

Perfect Partner Handgun: There are few things more fun than sharing a few boxes of 9mm ammo between your RRA CAR A4 and H&K’s new VP9 at the range.

This is a fully automatic rifle, although some of the other examples are single shot rifles. There is an argument that any rifle that uses pistol ammunition is technically called a sub-machine gun. This rifle differs from the typical sub-machine gun in that it has the long barrel and stock of a rifle. It's unclear to me if the technical difference between an assault rifle and a sub-machine gun matters for your story. You may want to be careful to avoid the term "assault rifle" unless you are willing to explain why you think that it is appropriate.

This is interchangeable ammunition, not ammunition of different lengths. The tradeoff here is that the rifle would always use the pistol ammunition. So its flight characteristics would match a really long barreled pistol rather than a target rifle. Of course, the kind of ammunition that an assault rifle (e.g. the M-16) uses is also a compromise. It's a smaller projectile and ammunition than used by battle rifles like the M-14. And the M-14 and other contemporary battle rifles used a shorter ammunition than the original Browning Automatic Rifle or the M1 Garand. Smaller and shorter means lighter. Lighter can be easier to carry, more rounds for the weight, or a combination of both.

The argument against using pistol ammunition is that it is shorter but still heavier. So if we take 210 rounds of M-16 ammunition as the typical amount a soldier carries, you might find that it would change to 150 or 160 rounds instead. And not fly farther or hit harder than the pistol. Only a bit more accurately, particularly at longer distances.

Different length with an automatic

There are no examples of this. Different length ammunition is used with single shot rifles. Interchangeable ammunition is the same length for both.

My quick guess as to the problem making the different lengths work in an automatic is that it would jam more often. It's adding more complexity to the rifle. It might actually be easier to reverse things, making a special handgun designed to fire rifle ammunition. It's unclear how useful that would be compared to a rifle with a long and short barrel combined with a removable stock.

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    $\begingroup$ Also the famous "Tommy Gun" used 45 caliber pistol ammo. $\endgroup$ – jamesqf Apr 9 '17 at 5:32
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    $\begingroup$ I'm no weapons expert, but according to the Wikipedia page the M-16 uses 5.56×45mm NATO or .223 Remington catridges. That's very different from the various .22 calibers (AFAIK, .22LR is one of the most common ones) and probably not common in pistols. $\endgroup$ – DarkDust Apr 9 '17 at 8:36
  • $\begingroup$ @jamesqf the 'Tommy Gun' is a submachine gun, a term invented by General Thompson when he developed it. $\endgroup$ – Sarriesfan Apr 9 '17 at 14:55
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    $\begingroup$ Not even remotely plausible. .22 caliber is the size of the projectile, not the size of the round. Their actual size varies wildly. The complete 5.56x45 NATO round, which the M16 fires, is 45mm long and 9.6mm in diameter at its thickest. .22 short is 10.7mm long and 7mm in diameter. .22 "Long Rifle" is 15.6 mm long. 5.56 NATO is rimless, .22 short and LR have a rim. 5.56 NATO is centerfire, meaning it has to be struck in the center by a firing pin. .22 short and LR are rimfire, meaning they need to be struck around the rim, probably by a hammer. $\endgroup$ – Schwern Apr 9 '17 at 18:19
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    $\begingroup$ @Brythan Everything you said is technically correct, but the implication that you can use .22 short and 5.56 in the same gun is not remotely plausible. That's what the question is asking, not whether there exists pistol and rifle rounds with the same bullet diameter. Your example isn't even in .22, it's 9mm, which is good because an M16 in .22 short or lr would be anemic. Otherwise this answer just says "use a submachine gun" and doesn't address the real question. (An AR-15/M-16 in a pistol caliber is a basically a submachine gun; an assault rifle uses an intermediate cartridge like 5.56 NATO) $\endgroup$ – Schwern Apr 10 '17 at 4:49
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pistol and assault rifle of the same bullet diameter, but different ammo length...just in case of running out of rifle ammo, one can load inferior and shorter pistol ammo in to rifle. (without caring any special converting gear)

Before going into the technical details, let's discuss why you wouldn't bother.

Pistols Are Not Useful On The Battlefield

Hollywood has given us an extremely exaggerated idea of what a pistol can do. A pistol is a personal defense weapon with an effective range of about 20 meters with moderate training, 30 if you practice. In any scenario where you're concerned about running out of ammunition, a carbine (ie. short barrelled rifle) is superior in almost every way except maybe weight. Pistols are inaccurate, underpowered, and have a low capacity.

A pistol has a very short barrel which cannot take full advantage of a higher power round, nor make it accurate. It has a low capacity 7 to 15 round magazine. The lack of a stock makes it difficult to steady, especially for follow up shots. Its short length means the rear sight and front post are too close together for accurate shooting.

In contrast a carbine has a longer barrel, generally has a full 20 to 30 round magazine, a stock to steady your aim, and the longer length provides more distance between the rear sight and front post to make aiming easier.

Ballistic Coefficient

Then there's the ammunition. Pistol ammo not only has less gunpowder, it usually has a smaller, lighter projectile than the equivalent caliber rifle round. This is bad news for accuracy, range, and stopping power. The ballistic coefficient is a measure of how well a bullet can overcome air resistance. A higher number means less drag which means it takes longer to slow down.

$$BC = \frac{mass}{diameter^2 * i}$$

As you can see, the more mass the better it will slice through the air.

For example, the FN Five-Seven fires 5.7x28, a 2 gram projectile at only 700 m/s. It starts slow, and slows down fast. In contrast 5.56x45 NATO, an intermediate round of nearly equal diameter, fires a 4 gram bullet at 900 m/s. It starts faster, and it can retain that velocity better.

There's little reason you'd want to carry a ton of pistol ammo. Instead, you'd carry a bunch of intermediate ammo, like 5.56x45 NATO and maybe a pistol and a few pistol magazines as a backup weapon in case your rifle jams or is lost.

Practical Alternatives

Firearms designers struggled with the gap between pistols and rifle rounds for about 100 years, and they've solved it pretty well in the last 50 by introducing intermediate cartridges like 5.56 NATO. The major realization was most combat happens inside 300 meters and a full power rifle round that can go out to 2000 meters was overpowered. This allowed the creation of smaller, lighter, shorter rifles with less recoil and more firepower that are plenty accurate. For special cases, you have a designated marksman.

This is why, for example, the M1 Carbine replaced the M1911 pistol or M1 Garand as the primary weapon for support personal in WWII. The M1 Garand was too big and heavy to be lugged around when it's not likely to be used. A .45 pistol is too inaccurate and only has 8 rounds. Currently the US military is in the process of replacing the M16 rifle with the M4 carbine; same basic rifle, same round, but one's a lighter, shorter carbine.

You'd either carry a carbine which fires an intermediate round, like the M4 carbine. Or use a pistol-caliber carbine like the Kel-Tec SUB-2000 or Beretta Cx4 Storm which gives you the benefit of a longer barrel, stock, larger magazine, and longer sight picture.

Another alternative is to use a rifle like the ubiquitous AR-15 that is amenable to conversion in the field. The AR-15 can be pulled apart without any tools, and you can swap out various parts to accept various calibers. This does mean you'll have to carry a different upper receiver, bolt, barrel, spring buffer, and maybe even trigger group, and you'll have to re-zero the rifle to the new bullet.

Not something you want to do in the middle of a fight, but not something you need an armorer for either. I just can't see any situation where, in the middle of a firefight, you suddenly find yourself with an overabundance of pistol ammo and no rifle ammo.

Probably your best bet is to use a single round, the FN 5.7mm, and carry both the FN Five-Seven pistol, and the FN P90 submachine gun. As you'll see below, the 5.7mm round is somewhere between a pistol and rifle round. Fired out of the longer barrel of the P90, it has a reasonable range and accuracy, and a 50 round magazine.

enter image description here

Source. Fans of the Stargate TV shows will recognize the P90 immediately.

The downside is FN 5.7 isn't a terribly popular round. While it is a NATO standard, it isn't used by much more than the Five-Seven and P90. If you're concerned about ammunition supply, go with something everyone is using. That will depend on where you are.


Caliber is the Diameter of the Projectile, not the Cartridge!

There's a common misconception that, for example, a 5.7mm round means the cartridge is 5.7mm. That's not true. For example, 5.56x45 NATO is 9.60mm in diameter at its widest; only the bullet, the tip of lead that actually fires, is 5.56mm (really 5.7mm, don't trust bullet names). FN 5.7x28mm isn't 5.7mm either, it's 7.9mm at the base.

enter image description here

Source. Left to right: 9x19mm, .40 S&W, .45 ACP, FN 5.7x28mm, 5.56x45mm NATO, .300 Winchester, 70 and 76mm 12 gauge shotgun shells. Note how very different in size 5.56 NATO and FN 5.7 are.

Cartridge and Bullet Geometry

Note that the bullets have a different geometry. The pistol cartridges are very short and cylindrical. This allows them to pack the most powder into the shortest possible space, something you want in a pistol which has to be compact. They fire a fat, flat nosed projectile. It isn't very aerodynamic, but pistols are inherently inaccurate; what happens after 50 meters doesn't really matter. It's all about making up for their relatively low velocity by throwing as much mass down range as possible.

Rifles don't have to worry as much about space, but they do have to worry about range and accuracy. They use a longer, larger diameter case to pack in as much powder as possible. Then it gets "necked down" to a smaller bullet diameter. This also helps feeding the longer cartridge by giving it a little ramp. A smaller, longer, pointed bullet has better aerodynamics and can fly further, faster, and flatter. These are known as (Spitzer bullets) introduced around 1900.

FN 5.7x28mm is somewhere in between. It's like a tiny rifle round, and that's what makes it so versatile. Compared to the 9x19mm, the ubiquitous 9mm pistol round it was designed to replace, it has far, far less bullet mass, 2g vs 7.5g, but it's fired at almost twice the speed, 715 m/s vs 400 m/s. Since kinetic energy is $mass x velocity^2$, despite being a third the mass both rounds have the same kinetic energy leaving the barrel. But its smaller diameter and aerodynamic shape means the 5.7 has better range and accuracy.

So while FN 5.7 might be a good choice if you want to settle on one round for everything, it will swim around in a gun chambered for 5.56 NATO.

Rimmed vs Rimmless

Revolver cartridges usually have a rim around the base significantly larger in diameter than the rest of the round. It's this rim that keeps the cartridge from sliding further into the chamber. This is why revolvers can handle cartridges of varying lengths. The downside is it's relatively weak and it's possible to tear the rim off when extracting in a high powered round.

Anything fed from a stacked magazine (ie. just about everything in the last 60 to 100 years) uses a rimless cartridge. It still has a rim, but it's the same diameter as the case, it doesn't stick out. The chamber is shaped to match the geometry of the cartridge and the cartridge fits snuggly into the chamber with minimal gaps, more on that below. The smooth geometry of the round means it stacks well in a magazine and feeds smoothly out of it, there's no rim to snag.

This is why, for example, there's a plethora of "ACP" pistol rounds. ACP stands for Automatic Colt Pistol. When semi-automatic pistols started appearing around 1900 they needed new, rimless cartridges to work with the new "automatic" magazine fed system.

Point is, you can't use a rimmed cartridge in a gun that expects rimless and vice-versa.

Centerfire vs Rimfire

Cartridges have a primer, a bit of explosive that will go off with hard whack. The hammer or firing pin strikes the primer which ignites the gunpowder which throws the bullet out of the gun. And, you should not be surprised to find out, there's two different ways this is done.

First is rimfire. That means the primer is around the rim. The hammer or firing pin strikes the rim of the cartridge. They're cheap, but they cannot be reloaded. For this reason, rimfire tends to be used on small cartridges where you're not throwing away a lot of brass.

Then there's centerfire which, you guessed it, has the primer in the center. These must be pin fired, but you can have a hammer which strikes the pin as in the Colt 1911. Centerfire is safer, produces a smoother detonation, and the case can be reloaded. Most importantly, they allow for a sturdier base which can withstand the higher pressures of rifle rounds. For this reason, nearly all rifle rounds are centerfire.

You can't use a rimfire cartridge in a centerfire gun, and vice-versa.

Bolt Locking and Headspace

AFAIK this has never been done, except for very small variations. For example, many rifles can take both 7.62x51mm NATO and .308 Winchester, though there may be malfunctions because of the different powder loads and chamber pressures.

The primary problem has to do with the bolt locking mechanism and headspace. Any breech-loading gun (ie. gun for the last 150 years or so) has to have some way to open the breech so a round can be loaded, and then seal it so the round can be fired without high pressure gas leaking out.

There are many, many ways to do this, but it usually involves a "bolt" which moves back to extract a spent round, and forward to strip a fresh round off the magazine and shove it into the chamber. The bolt is locked in place: rifles usually have a rotating bolt with locking lugs (just like a bolt-action rifle), while pistols usually have a less robust but shorter mechanism based on recoil.

The bolt face is flush against the base of the round. This is both to lock the breech so gas cannot escape backwards, and also because the firing pin is usually in the bolt face.

Point is, you can't have a gap between the bolt face and the round, and the bolt must be securely locked. That means the round must be of a specific length and geometry for that chamber. Too long and the bolt won't lock. Too short and the round won't fit snugly in the chamber, when the round is chambered it might slide too far forward resulting in a gap between the bolt and bullet base that could cause a rupture when the bullet is fired. Or when the firing pin strikes the base the bullet might slide forward resulting in a light strike and the bullet not firing.

Cycling The Action

The next problem is getting any semi-automatic gun to reliably cycle with such wildly different powder loads and projectile weights. There's two major ways semi-automatic and automatic weapons are cycled: gas and recoil. Gas uses some of the gas from the exploding gunpowder tapped from a port at the end of the barrel. Recoil uses the recoil force of the projectile. Both work only within a specific range of forces.

Gas-operated weapons can usually be adjusted by tuning their gas port: how much gas gets vented from the barrel into the mechanism. This adjustment can usually be done in the field with no special tools. Recoil-operated weapons usually require changing the recoil spring.

Neither are designed to handle such wildly different amounts of gas and recoil as between a pistol and intermediate cartridge. Instead it's about variations in ammunition quality, environmental conditions, and how well maintained the weapon is.

You'd have to make a lot of compromises to have a semi-automatic weapon that would cycle reliably with pistol and intermediate cartridges with just field adjustments.


In any situation where ammunition supply matters, you're not going to be using a pistol as anything but a backup weapon, so carrying a bunch of pistol ammo doesn't make sense. Go with a carbine instead: it's more accurate, and has a greater magazine capacity. If you really want to be flexible, carry an AR-15 and a conversion kit. Or chuck the conversion kit and use the saved weight to carry more rifle ammo.

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    $\begingroup$ Very informative answer. I like the last sentence the most, just use the weight saved to carry more ammo. $\endgroup$ – Kilisi Apr 9 '17 at 6:13
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    $\begingroup$ +1 for the detailed analysis, including addressing one of the common claims that the FN 5.7mm, despite having fairly nice ballistic properties, still comes out with a relatively low muzzle velocity (better out of a P90, but not by that much). Though I did feel like it deserved a little bit of recognition that it didn't get when mentioning pistol vs. carbine magazine capacities, since a stock FN Five-seveN fits 20 rounds in a regular-sized clip, and has a relatively small extended clip for 30 rounds. But that's a minor nit-pick - very informative and nice answer overall. $\endgroup$ – mtraceur Apr 9 '17 at 9:31
  • $\begingroup$ The conversion is actually a lot easier than you make it sound. I have two complete uppers for my AR at home: One in 5.56, and one in 300 Blackout. All I have to do pop the takedown pins out and swap them - it's a completely no-tools operation that can be done in seconds. As for re-sighting, the optics stay attached to the upper, so they don't need to be adjusted. That being said, I'm not exactly a competition shooter, so there could be some small inaccuracy in there. $\endgroup$ – UIDAlexD Apr 11 '17 at 19:42
  • $\begingroup$ @UIDAlexD I agree it's an easy procedure, if you know what you're doing. I picked the AR-15 because it's so easy to takedown. But you have to have the parts with you (weight & planning), and it's not something you want to plan on doing in a firefight. As for the sights, you'd need to realign your sights for the different round with different ballistics. 5.56 and 300 Blackout fire pretty flat out to 200 meters, so you can get away without adjusting your sights, but a pistol round will have significant drop. You'll probably be ok inside 100 meters. $\endgroup$ – Schwern Apr 11 '17 at 21:08
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This is all going to depend on your definition of "reasonable". A pistol is normally fired in one hand. This limits its weight to about 2 to 2 1/2 pounds, both in terms of aiming accurately and recoil. An assault rifle is intended to use both hands, and a weight of 6 to 8 pounds is a reasonable number. So, in rough terms you can expect an assault rifle to weigh about 3 times what a pistol does.

This in turn suggests that an assault rifle round will have a momentum (mass times muzzle velocity) about 3 times that of a pistol, so an assault rifle which fires a pistol round will be rather underpowered compared to assault rifles which do not. The recoil of a weapon is (to a first approximation) proportional to the mass of the weapon for a given round. Furthermore, since a pistol round will be designed for complete powder combustion within the rather shorter barrel of a pistol, an assault rifle with a normal barrel length will be grossly underutilizing its barrel length, and will be larger and heavier than it needs to be.

The thing is, you haven't defined "reasonable". If the issuing military decides that the benefits of a single cartridge outweigh the drawbacks of reduced performance, that is entirely up to them.

The P90/FiveseveN combination is a good example. The small caliber permits decent energy at the cost of momentum, so the pistol is adequate. The P90, on the other hand, was intended for use by those not ordinarily in combat, such as vehicle drivers, and at short range. It's been adopted by police-type units, but it's not generally considered powerful enough for front-line troops. The same sort of calculation is likely to recur for any use of a pistol round in the assault rifle role.

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  • $\begingroup$ But as I understand it this just means that the transmitted energy when used with pistol rounds would be similar to a pistol? Otherwise it would still perform better than a pistol and would make it unnecessary to carry a pistol around. $\endgroup$ – Michael Apr 8 '17 at 20:19
  • $\begingroup$ @Michael - Close, but there are still advantages for the assault rifle. The longer barrel and associated sight radius can greatly increase the maximum range in terms of accuracy, and the two-handed grip allows accurate/effective automatic fire. Plus, for troops who are not normally combatants the reduced bulk of a pistol keeps it from interfering with their normal duties as much. $\endgroup$ – WhatRoughBeast Apr 8 '17 at 20:24
  • $\begingroup$ Yes, that’s what I tried to summarize with “Otherwise it would still perform better than a pistol”. Or is the usage of “otherwise” not correct in this context? English is not my native language. $\endgroup$ – Michael Apr 8 '17 at 20:27
  • $\begingroup$ Pistols and assault rifles are used for very different purposes, so "perform better" does not have the same meaning when comparng the two. $\endgroup$ – WhatRoughBeast Apr 8 '17 at 20:30
  • $\begingroup$ Note that firing pistols with one-hand went out of style decades ago. If you want to hit anything, you use some variation of the two-handed Weaver stance. It just doesn't look as cool. $\endgroup$ – Schwern Apr 9 '17 at 1:51
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Probably

It would not be very practical, you could have a weapon with 2 different bolt carrier group that could be moved into place like the cylinders in a revolver. Each could accommodate a different length ammunition. The drawback is the weapon may not cycle properly when switching to the lower power pistol rounds if gas or spring operated. Another drawback is the weapon becomes more complex.

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OK, so the problem is using current firearms and brass casings. Go to a caseless design and it would be doable. Why? Most pistol rounds are straight walled cases, with a rim (mostly revolvers) or rimless (mostly automatics). Most rifle ammo are "bottle neck" designs, where the base (with or without rim) and lower half of the casing are a larger diameter, and then they neck down (at various degrees, depending on the round) to the bore diameter.

So a traditional pistol round isn't going to work with a traditional bottle neck rifle round. There would be nothing to support the case walls, you'd have case blowouts and all other sorts of kaboom-like experiences. Not good.

All that said...

Go to an older design, like revolvers and lever action rifles (1860/70s technology) and it already exists, either using same ammo or using the "special" version of magnum pistol calibers - 38 Special and 357 magnum are exact same, but the case length on the 357 is .1" longer. Same with 44 Magnum and 44 Special, several 32 calibers (32 long, 32 h&r magnum, 327 federal), etc. More recently Ruger has made semi-auto and bolt action carbines and rifles in 357 and 44 magnum.

Of course, rifles and pistols can be designed to use the same round from the first drawing on... This is the case of the PS90 and FN57 you mention.

There are also ARs and AR-like rifles/carbines chambered for pistol rounds, some using the same magazine as a popular pistol in that caliber (Glock, Beretta, etc).

You can also create what the US Government defines as a pistol by not putting a full length stock and a shorter barrel on, in a very specific manner (otherwise you end up creating a short barrel rifle, that while cosmetically and functionally identical is a very restricted NFA item and requires a $200 tax stamp, extended back ground check, etc). There are plenty of AR and AK based pistols.

Finally lets not forget the various single shot rifles chambered for pistol calibers, and the "specialty" pistols chambered for various bottle neck rifle rounds. Of course, almost all of these are single shots.

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  • $\begingroup$ From what I've read (I'm no expert), pistol and rifle cartridges use different types of powder. The pistol powder has to be faster burning because of the shorter barrel. Using pistol powder instead of rifle powder can apparently have undesirable consequences, e.g. bulletin.accurateshooter.com/2015/01/… $\endgroup$ – jamesqf Apr 8 '17 at 19:52
  • $\begingroup$ Welcome to WorldBuilding! Interesting answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Apr 8 '17 at 20:41
  • $\begingroup$ @jamesqf - in general that is true, but there is almost always more powder than needed in any given pistol round for anything approximating a "normal" barrel length (4-5" for a full size pistol like police or military would carry) and a longer barrel can significantly increase velocity - sometimes. Do a google for "ballistics by the inch" and you can see what effect barrel length has on quite a few different calibers. $\endgroup$ – ivanivan Apr 8 '17 at 23:04
  • $\begingroup$ @ivanivan: That's just the opposite of what I was thinking about - that using fast-burning pistol ammo in a rifle would cause overpressure, with results like what's shown in the link. Of course actual pistol ammo might have a good deal less powder than a reloaded rifle cartridge. $\endgroup$ – jamesqf Apr 9 '17 at 5:28
  • $\begingroup$ @jamesqf - powder is measured by weight in grains - 15.43 per gram. A 9mm load will have 5-10gr of powder - how much depends exactly on which one, its burning rate, and its volume by weight. 223 Remington or 5.56 NATO both share the same case head size with 9mm, but are longer and then of course bottle neck down to .224" diameter. A typical 223 load will have 20-25gr of powder. More case volume to fill (important!) and a slower burning rate. No matter what firearm the ammo is used it, any given caliber will use the SAAMI specs for chamber dimensions and max chamber pressure. $\endgroup$ – ivanivan Apr 9 '17 at 17:51
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No, because you said assault rifle, which implies semi-automatic. For a variety of reasons, but mainly because of rim lock, almost all practical semi-auto weapons use rimless cartridges. Rimless cartridges headspace off the shoulder or neck of the cartridge. So even if length were the only difference between the rifle and pistol cartridges, you'd still need to swap barrels for proper headspace. So while this is technically possible, it wouldn't be very practical. Instead of lugging around an extra barrel on the off chance of needing to fire pistol ammo, a soldier would be better off carrying more primary ammo.

This might actually be practical if you weren't asking about semi-automatic rifles. There are carbines that fire pistol ammo, and the primary ammo for the rifle could be loaded hotter than normal pistol ammo. (Although then you would have to be careful not to accidentally load the rifle ammo in a pistol.) You could also have a bolt action rifle that fires straight-walled rimmed cartridges (which headspace off the rim) and can fire shorter pistol cartridges without modification. There would be no risk of loading the rifle cartridges into a pistol.

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  • $\begingroup$ Doesn't the FN P90 (same cartridge as the FN Five-seveN mentioned in the question) achieve both semi-auto and full-auto just fine using a rimmed cartridge? $\endgroup$ – mtraceur Apr 9 '17 at 9:34
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    $\begingroup$ @mtraceur 5.7x28 is rimless. $\endgroup$ – Kevin Krumwiede Apr 9 '17 at 9:44
  • $\begingroup$ Ah, sorry, I'm not very up-to-speed on various cartridge/case/etc terminology. I was getting "rimmed" confused with whatever it's called where the bullet casing gets less wide towards the bullet. I've gone ahead and +1'ed your post, now that I understand I was just unaware of what I was talking about. Question: Besides the 5.7x28, which is rimless but has use in both pistols and things like the FN P90, are pistol cartridges predominantly rimmed in general? $\endgroup$ – mtraceur Apr 9 '17 at 9:52
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    $\begingroup$ assault rifle means select fire, with options for burst and semi auto. (you may know this, but most readers don't, and your first sentence makes this unclear). $\endgroup$ – atk Apr 9 '17 at 15:06
  • $\begingroup$ @mtraceur - you are thinking of bottle neck rounds. With handguns, almost all revolvers use rimmed cases, and almost all semi-autos use rimless or semi-rimmed (essentially rimless w/ a slightly wider base than the other side of the extractor groove in the case). With long guns, typically rounds originally designed for single shot or being fed from a tube magazine will have a rim, rounds designed to be fed from a box magazine are rimless but there are several exceptions to both. $\endgroup$ – ivanivan Apr 9 '17 at 17:57
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Not really as you have phrased the question.

By definition assault rifles use high velocity but small calibre rounds. The rational being to give a weapon which can provide effective and accurate fire over moderate distances (say about 300m) but still retain good accuracy and handling characteristics at close quarters and in confined spaces.

By contrast pistols and sub machine guns tend to use large calibre, low velocity rounds which provide good accuracy and handling at short range but rapidly drop off as range increases.

There is also the fact that pistols and rifles tend to use a different operating mechanisms. Most pistol calibre weapons operate by blow-back where the force reaction force of the cartridge on the bolt pushes the bolt backwards and cycles the action whereas automatic rifles tend to use a gas piston arrangement.

There is also the consideration that pistol calibre rounds are now seen as increasingly less viable as military weapons, except as lightweight secondary sidearms, partly due to the proliferation of body armour which is very effective against pistol rounds.

What is increasingly common is a radically shortened version of the standard infantry assault rifle, for example the L22 carbine version of the L85A2 used by the British Army and issued to aircraft and vehicle crews. There are also a number of purpose built weapons with a SMG type barrel length but assault rifle ammunition type.

Equally short barrel assault rifles are generally superior to SMGs in performance with very few drawbacks and having the advantage of reducing the number of different ammunition types required. Even specialist units which like counter terrorist units of police and military which retained SMGs much longer than regular infantry have now mostly switched to some form of assault rifle chambered for an intermediate round.

If anything the trend in the military has been to go back to larger rounds in the 7-8mm range for increased effectiveness at longer ranges, especially in light of experience in the Middle East.

Similarly the only real logic for carrying pistol ammunition is in very limited quantity for a secondary weapon which you would probably only use if your primary weapon was lost or inoperable.

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Possible? Yes
Doable? Yes
Better idea? Of course.
Use the same magazines that will fit the gun and rifle.

But for the quick answer: Rock River Arms LAR-9 CAR A4 and KH VP9
CMMG Mk9 T and CZ P-09
You probably noticed the number 9 is reappearing. It's 9mm. Those wacky germans in the XIX century figured out it would be cool if the same bullet would fit in the carbine, gun and revolver. Oh, and if the gun could be easily converted into a carbine (hence the Mauser broom).

When you consider using shorter ammo for the pistol in your rifle remember that this is actually useless from a logistic point of view. Yeah, the ammo crates carry more ammunition with pistol ammunition, but they also weigh more. And it's better to stock and carry better ammo. So, from a soldiers point of view it's better to have 3 magazines for their rifle on them than 4 magazines for their pistol.

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For your information: Hi-Point firearms makes both a pistol and a carbine that not only fire the same round but also share the same magazine, available in .40, .45 and 9mm. American made, lifetime warranty and affordable.

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  • $\begingroup$ Welcome to WorldBuilding! This looks interesting. Do you have any sources you could cite and summarize to underline your point? Right now your answer seems valid, but a bit short and might be better as a comment. You can comment once you reach 50 reputation. You can edit with the little "edit"-button at the end of your answer. Please provide more information. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$ – Sec SE - clear Monica's name Apr 9 '17 at 9:21
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    $\begingroup$ And it kinda looks like this is advertisement. You might want to expand on the general points to differentiate yourself from the company, otherwise this looks like spam advertisement. $\endgroup$ – Sec SE - clear Monica's name Apr 9 '17 at 9:23
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    $\begingroup$ It's not spam, but directly answers the question. It should sound less like an ad though (delete the last sentence) and link to the product. The poster's association can be stated below the main text. $\endgroup$ – JDługosz Apr 9 '17 at 15:14

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