.22 LR Question

[KJS]

I'm a novice, so please excuse what will likely sound like a totally ignorant question.

How is it that .22 LR works just fine in semi-auto pistols, while other ammo with a rim that sticks out wider than the rest of the case (revolver ammo) doesn't work in semi-autos designs?

Other than the Desert Eagle in .44 Mag & Baby Eagle in .357 Mag I can't think of any semi-autos that attempt to fire rimed cartridges designed for use in revolvers. It was my understanding this isn't done by other companies because such revolver cartridges simply don't feed well in autoloaders. So much so that Sig Sauer created their .357 Sig to imitate the ballistics of a .357 Mag in a semi-auto cartridge.

Yet, .22 LR that has a rim sticking out wider than the case just like any other revolver ammo works fine and is frequently used in semi-auto pistols.

 

[SDC]

It's possible to do it, but it's a lot of work; besides the Desert Eagles, a number of semi-autos have been built in rimmed cartridges like 38 Special, 357 Mag, and so on, but there are a couple of problems you have to work through to get them reliable enough that people will want to buy them in the first place.
22s don't require as much work because they're straight blowback, meaning they can be built with a spring and bolt/slide light enough that they're still easy to cock, but larger calibres need to have that energy left over after unlocking the breech (either by rotating a bolt, dropping the barrel, pivoting a locking arm, etc.), and conserve that energy over a longer bolt/slide travel.
There are very few sharp edges on most 22 LR ammo, so the rounds will generally slide past one another without much trouble, as long as you don't load them in a "rimlock" situation, where the rim of the top round is behind the rim of a lower round.
The larger diameter of centrefire rounds also means that the rounds "stack up" in a sharper radius more quickly than with 22s, which causes more problems in designing magazines and a comfortable grip shape; you can get an idea of what I mean by laying 10 rounds of 22 LR and 10 rounds of 38 Spl on a table, and seeing how large the curve becomes with the larger-calibre rounds.
The sharper edges on centrefire rimmed rounds also makes them much more likely to snag on any little burr or edge inside a magazine or feedway. To get around this to some degree, they've actually produced semi-rimmed ammunition for some uses, with a smaller-diameter rim to allow for more reliable operation.

 

[highorder]

The overall length of rimmed rounds precludes chambering them in most semi-auto designs, in addition to the good information from SDC.

 

[KJS]

Thanks SDC for that explanation. I don't know enough to fully understand it, but I see what you mean about the arc shape that is produced when you try to stack rimmed cartridges. The rim prevents a stack that is nice & straight.

The overall length of rimmed rounds precludes chambering them in most semi-auto designs, in addition to the good information from SDC.

I have noticed that semi-auto rounds like .45 ACP do stand out as short & fat when compared to .357 Mag that stands tall & lean by comparison. Are you saying cartridges as long as a .357 Mag would simply make for a grip that's too large to be comfortable for many shooters? Or does length have some other effect I'm not aware of?

 

[highorder]

Are you saying cartridges as long as a .357 Mag would simply make for a grip that's too large to be comfortable for many shooters?

You're right on the money.

 

[bigfatdave]

.32acp is semi-rimmed, and is occasionally subject to rimlock if your ammo is shorter than the magazine internal length (happens more with HP designs, can be overcome by careful loading and sometimes by adding a spacer to a mag, making it HP only)

Most semi-auto pistol ammunition is designed to fit into the chamber with the headspace defined by the mouth of the casing, while most revolver ammunition headspaces based on the rim. .22S/L/LR and some small pistol calibers headspace on their rims (some of the rims are so small you have to check against a straight edge or flat surface).
This leads to an interesting difference you have probably noticed - revolvers can safely fire ammunition which is shorter than their design ammo, so long as it is the same rim/case/bore diameter, while semi-auto pistols cannot. A side-effect is that case length precision in revolver ammunition is not as critical as in S.A. ammo (although revolver ammunition is fairly precise anyway, just because ammunition in general is fairly precise dimensionally).

Generally, if a gun is designed to fire a rimmed cartridge and load them into the chamber from a box magazine, the magazines get more curved with larger capacity, creating a "banana clip" effect, which is not desirable in pistols, obviously. Some (Walther P22) .22LR pistols keep the rounds staggered and crossing in an attempt to avoid the curve, some (Ruger MKI/II/III) are sloped quite a bit to allow a slim, straight magazine ... scaling those designs up is possible, but impractical considering the fine choices offered in S.A. ammunition designs.

You mentioned the .357sig round, but if you didn't know, there are a number of bottle-necked pistol cartridges on the market, from the venerable 7.62x25mm Tokarev, to the modern .32NAA (essentially a .380ACP case necked down to .32).

 

[bigfatdave]

Are you saying cartridges as long as a .357 Mag would simply make for a grip that's too large to be comfortable for many shooters? Or does length have some other effect I'm not aware of?

Yes, the grip would get overly long (front-to back).
Also, remember that the action would have to cycle over a longer length, I'm not sure if that would cause reliability issues, but I imagine such a long cycle would eat up springs and wrists, at the least.

 

[KJS]

This leads to an interesting difference you have probably noticed - revolvers can safely fire ammunition which is shorter than their design ammo, so long as it is the same rim/case/bore diameter, while semi-auto pistols cannot.

Yeah, I've noticed. Like my .357 that can fire .38s (and has been forced to due to Obama-induced ammo shortages that make .357 far harder to find than .38). Or a .44 Mag that can fire .44 Special or a .460 Mag that can fire .454 Casull or .45 Colt or a .454 that can fire .45 Colt. Or for the ultimate in different ammos, one can buy a Judge and fire a magnum or standard size .410 shotshell or .45 Colt.

 

[PO2Hammer]

Disregard