How a cartridge headspaces — what stops it at the right depth when the action closes — decides what it can safely feed. This set contains ordinary rimless autoloader rounds, one semi-rimmed transitional design, and a revolver cartridge that works on a principle nothing else in production shares.

Sorting by mechanism prevents most of the expensive mistakes.

The revolver with no cylinder gap

7.62 x38r was designed with a case mouth that extends past the bullet, and the Nagant M1895's cylinder moves forward on firing so that mouth enters the forcing cone. It seals the gap every other revolver leaks gas through. Ammunition not loaded to that geometry defeats the mechanism, which is why there is no generic substitute.

Rimless, for a box magazine

7.62x25 ammo is a rimless bottleneck built to feed from a Tokarev's magazine and, later, a submachine gun's. 32acp is a rimless straight-wall from the same pocket-pistol tradition as its .25 sibling. 9x23 winchester, super 45 and 10mm auto ammo are all rimless straight-walled autoloader rounds, and so are the brand lines loaded in the 10mm case — underwood 10mm powder, federal 10mm auto ammo and sig sauer 10mm fmj ammo feed exactly like any other 10mm because they are 10mm.

The semi-rimmed transitional design

9mm long keeps a slight rim on a magazine-fed case, a convention common in European autoloaders around 1903 before fully rimless designs took over. It feeds from a box magazine but headspaces partly on that rim — neither fully one thing nor the other.

Why the look-alikes fail

Most errors in this aisle are dimension errors, not rim errors: the 7.62x25 and 7.63x25 Mauser are near-twins at different pressures; the 9x21, 9x23 and 9x19 share a bullet and nothing else; .45 Super and .45 ACP share every external dimension and differ only in what the case can take. The case base is the first check; the case length and the stamp on the gun are the second and third.

Never substitute a similarly-named cartridge without confirming case length, rim design and pressure rating against your firearm's chamber markings or owner's manual, and verify current stock and any state shipping restrictions before ordering.