Calibre

A calibre (or caliber in American English) is the diameter of the barrel of a gun. For example, a rifle with a diameter of 0.22 inch is a .22 cal. Guns that have a particular calibre are designed to fit and use bullets of the same calibre.

Types of callibre

Callibre is measured in inches, not millimetres. Common callibres include:

Common calibers in inch and their metric equivalents^{[1][2][3][4][5]}

Inch caliber	Metric caliber	Typical bullet diameter
.172	4 mm	0.172 in
.20, .204	5 mm	0.204 in
.221	5.45 mm	0.221 in
.22	5.6 mm	0.223 in
.224	5.7 mm	0.224 in
.243	6 mm	0.243 in
.25	6.35 mm	0.257 in, 6.35 mm
.26	6.5 mm	0.264 in, 6.7 mm
.27	6.8 mm	0.277 in, 7.035 mm
.284	7 mm	0.284 in, 7.213 mm
.308	7.62 mm	0.308 in, 7.82 mm
.311	7.9 mm	0.311 in, 7.92 mm
.312	7.94 mm	0.312 in, 7.94 mm
.323	8 mm	0.323 in, 8.20 mm
.327	8 mm	0.327 in, 8.30 mm
.338	8.6 mm	0.338 in
.355	9 mm	0.355 in
.356	9 mm	0.356 in
.357	9 mm	0.357 in
.363	9 mm	0.363 in
.365	9.3 mm	0.365 in
.375	9.5 mm	0.375 in, 9.53 mm
.40	10 mm	0.400 in
.44	10.9 mm	0.429 in
.45	11.43 mm	0.451–0.454 in
.50	12.7 mm	0.510 in, 12.95 mm