Whether you want to select the right bullet for handloading, or identify what caliber or cartridge that unmarked barrel takes, this chart ought to make things a bit easier.

Key

ACP = Automatic Colt Pistol

CF = Center Fire

mm = Millimeter

perc = Percussion

Rem = Remington

Win = Winchester

WCF = Winchester Center Fire

WSL = Winchester Self Loading

Cartridge/Caliber	Barrel Dia.
All .22 Rim Fire	.222"

.22 Rem Jet	.22"

Most .22 CF	.224"

.243/6mm	.243"

.25 caliber	.257"

.264/6.5mm	.264"

.270 caliber	.277"

7mm/284	.284"

.30 caliber rifle	.308"

.30 Luger	.310"

.30 Mauser	.309"

.31 perc Colt	.313"

.32 ACP	.311"

32/20, 32 S&W	.312"

7.65mm	.311"

.303 British	.313"

8mm Mauser (J)	.318"

8mm Mauser (S)	.323"

.32/40 & Special	.321"

.33 WCF & .338	.338"

.348 Win	.348"

.351 WSL	.352"

9mm Luger	.354"

.38 Special/.357	.357"

.36 perc Colt	.375"

.380 ACP	.355"

.375 Magnum	.375"

38/40	.400"

38/55	.379"

.401 WSL	.406"

.405 Win	.412"

.41 Colt	.406"

.41 Magnum	.410"

44/40	.427"

.44 perc Rev	.451-.457"

.44 S&W American	.436"

.44 Russian, Special & Magnum	.430"

.444 Marlin	.430"

11mm Mauser	.446"

.45 ACP	.451"

.45 Colt	.452-.454"

.455	.457"

45/70	.457"

.458/460	.457"