The single most useful thing to know about an unfamiliar cartridge is how it headspaces: what stops it at the right depth when the action closes. That comes down to the base of the case, and it sorts this whole list into three groups.

A revolver, a lever-action rifle and an autoloading pistol each want something different from that base.

Rimmed, for the revolver

A revolver cylinder headspaces on the rim and the extractor star grips it, so revolver cartridges are almost universally rimmed. 357 max, 327 federal magnum, 32 long ammo, .455 webley, 38 colt ammo and 41 mag ammunition all belong here — Victorian service rounds and 1980s magnums alike — and none of them will feed reliably from a box magazine.

Rimmed, for the lever gun

44 40 wcf is rimmed for a different reason: a tubular magazine stacks rounds nose to primer, and the rim keeps each case from sliding forward onto the primer ahead of it under recoil. That the same round also ran in the Colt Single Action Army was a commercial bonus, not the design driver.

Rimless, for the magazine

An autoloader extracts every case on a fixed timeline, so it wants a case that headspaces on the mouth or shoulder and stacks cleanly. 30 Luger ammo is the textbook case, a rimless bottleneck designed alongside the Luger's toggle action in 1898. .30 super carry is a straight-walled modern rimless design built to feed from a slim double-stack magazine, and 4.6mm ammo is a rimless bottleneck built for a PDW's box magazine.

Why the look-alikes fail

Most ordering mistakes in this category are rim mistakes. A rimmed round of the right diameter will not feed from a magazine; a rimless round of the right diameter will fall through a revolver cylinder or fail to eject. Confirming the base before confirming the bore saves the return.

A quick check on a listing photo

A visible lip wider than the case body is a revolver or lever-gun round; a smooth taper to the mouth is a magazine-fed round. It is not a substitute for reading the firearm's own stamp, but it catches an obviously wrong listing before checkout.

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