Definition: The word "calibre" (US spelling "caliber") is derived from a French term denoting social standing or quality. As applied to firearms, it describes the internal diameter of the barrel. In a smooth-bore weapon, this is a straight measurement from an internal surface to the opposite internal surface. In the case of a rifled barrel (i.e: a barrel with grooves cut into its internal surface for the purpose of imparting spin to a projectile fired from that barrel), there are essentially four conventions:
- Land-land (i.e. raised portion to raised portion), e.g. .30"/7.62mm, .303", 5.56mm etc.
- Groove-groove (i.e. sunken portion to sunken portion), e.g. .308", .223".
- Bullet diameter (if different from both of the above), e.g. 10.4x38R ( smaller than l-l measurement), .303" Savage (bigger than gr-gr measurement).
- Random (often overstated), e.g. .38 Spl (.357" nominal gr-gr), .44 Magnum (.429" nominal gr-gr).
About the only way it is not measured is from the bottom of a groove to the top of the opposite land (the ungrooved portion of the barrel interior).
Method of expressing calibres: Historically, British and US calibres have been described in inches and Continental calibres in millimetres. Examples are .300" (US .30Cal) = 7.62mm; .303" = 7.7mm; .5" (US .50Cal) = 12.7mm.
Inconsistencies: One should note that stated calibres of weapons are largely a matter of convention and not accurate conversion. Ian V. Hogg, in "GUNS and how they work", points out that .223" (the well-known NATO standard 5.56mm) actually works out at 5.66mm (.223 x 25.4 = 5.66... which is ironic really, because the nominal bullet diameter is .224", or 5.69mm). Similarly, whilst everyone associates the tag .357" with the incredibly high-powered Magnum round of "Dirty Harry" fame, all .38" revolvers measure .357" across the bore. (The .357" Magnum simply loads a longer cartridge, and the revolver frame is strengthened accordingly.)
- Room for confusion
Just to make life interesting, the lengths of artillery guns are quoted in "calibres", meaning the barrel is so many times the bore of the barrel. This is helpful in one sense, you know that a 36 calibre barrelled SPG has a much longer gun than a 20 calibre one and so more likely to check its shell further from you - or further towards you.