This comes up now and again, and I haven’t posted a firearm entry for some time, so here’s a new one.
There are several calibers that sound like they should be interchangeable, but the truth is, there are slight differences that can lead to disaster for your fictional characters, and your real life ones.The two most common offenders are the .38 Special/.357 Magnum and the .44 Special/.44 Magnum. A lesser known issue comes with the .40 Smith and Wesson and it’s mixing with the 10 mm, 9mm, and .357 SIG.
Let’s start with the .38 Special. This is most commonly found in revolvers and dates back to the end of the 1800’s. It’s the most commonly found pistol cartridge in just about police story through the 70’s. In its day, it was seen as a powerful round. It comes in behind the 9mm, and even most loadings of the .380 ACP today.
The .38 Special has been improved with the addition of the .38+P and .38 +P+. These are the same size as a standard .38 Special, but run much hotter. Firing a +P in a gun not rated for it can lead to damage and injury. Any gun chambered in .357 Mag can handle them all with ease.
The .38 fires a bullet that is .357 inches in diameter. This is identical to the bullets fired by the .357 Magnum. In fact, the same bullets can be fire in both.
Note BULLETS, not loaded rounds. The .357 Magnum has a case which is slightly longer than that of a .38 Special. This is specifically to prevent the much more powerful round from being loaded in the structurally weaker .38 frame. In fact, the .357 has about twice the pressure of a .38 Special.
Putting a .357 Mag in a .38 Special, even it fits somehow, can result in an overpressure that will explode the cylinder. The character shooting may come away with all their fingers. Or not.
The .44 Special is also an older, lower powered cartridge, and many of the same applies to its relationship with the .44 Mag, with one exception. The .44 Special is slightly bigger around than the .44 Mag.
EDIT: The above was pointed out to be incorrect. The slugs fired from a .44 magnum can indeed be used in a .44 Special, but the case length will prevent putting a .44 Magnum in a .44 Special. The outer diameters of the cases are the same, so there would be no issues in firing a .44 Special round in a 44 Magnum.
The 357 SIG is an odd round. It’s a pistol round that has a bottle shape like a rifle. The case is the same size a .40 S&W, and the bullet is the same as a 9mm (0.355 inches).
Because the cases are the same size, a .357 SIG round can be loaded into a .40 S&W magazine and chambered into the pistol with no difficulty. When it fires, the bullet will have about 0.5 mm (about 1 mechanical pencil lead) all the way around. Accuracy will suffer, but it probably isn’t enough room to get the bullet crossways in the chamber or barrel. It also isn’t likely to have enough pressure to cycle the slide. Worst case of a .357 SIG in a 40 S&W is a single shot that can’t shoot straight.
You can’t put a 40 S&W in the chamber of a .357 SIG.
You can put a 9mm in the chamber of either. The 9mm case is slightly smaller than the .40 S&W, in diameter and length. If one finds its way into the chamber of a .40 S&W, it can actually slide up far enough to allow a second one to chamber behind it. This will almost certainly result in an exploded gun. The first round will fire and the bullet will hit the back of the lodged round. This is too much mass to push forward as the pressure of the burning powder builds, so the pressure will find another way out. This will cause the slide to come back much harder than normal. About this same time, the second round (the one fired) may set off the primer of the first one, causing it to discharge as well. Because the barrel is a little bigger, the bullet won’t really be restricted, so this pressure won’t be that big of a deal. The case being shoved backwards might be.
Bottom line is that not all rounds are equal. If your character takes a gun off someone, and a magazine off another guy, the magazine might fit, the rounds might not.*
If you need a gun to fail, or to become a sudden risk to the shooter, incompatible ammo is one way to go.
*This is a peeve of mine in some action movies. The “elite” team of good or bad guys all have different weapons. I don’t mean, one shotgun, three assault rifles, and 4 SMG’s, which may be tactical choices. I mean the three assault rifles, are an M-16, AUG, and AK-47, all of which use different magazines, and one of which is a different caliber. That means if one guy falls, no one else can use his stuff, but it still needs to be disposed of so the other side can’t use it.