The base of a cartridge case decides what action it can feed. On this shelf that produces a rare thing: five different answers on one list, including one design most shooters never encounter.

Sorting by base takes a minute and prevents the one dangerous confusion here.

Rimless, for a box magazine

9mm ammo and .45 ammo are the archetypal rimless autoloader rounds; every 9mm and .45 ACP pistol headspaces them on the case mouth and stacks them in a magazine. The brand lines follow the cartridge: 9mm ammo blazer, federal 115 grain 9mm, hornady 9mm critical duty, subsonic 9mm rounds and winchester hollow point 9mm are 9mm Luger and feed exactly like it, because they are it.

Rimmed, for a revolver

.45 long colt ammo is the rimmed exception, the 1873 revolver round that headspaces on its rim and ejects with a star extractor. It will not feed from a magazine, and a rimless .45 will not headspace in its cylinder without moon clips.

Rebated, so a big case fits a small bolt

50ae uses a rebated rim — a rim smaller than the case body — so the Desert Eagle's bolt face, sized for the .44 Magnum, could handle a fatter .50-caliber case with only a barrel change. It is the only rebated-rim round most handgun shooters will ever see.

Rimless and bottlenecked, for a rifle

remington 17 is a rimless bottleneck built for a bolt-action rifle's magazine, a .223 case squeezed down to a .17 bullet. It has no business in a pistol and appears on this list only because the store's catalog put it here.

The one confusion that matters

A .45 ACP and a .45 Colt share a bullet diameter and nothing else — rimless versus rimmed, autoloader versus revolver. Read the base, then the gun's stamp.

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