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Basic Information

In the context of firearms the word caliber (also "calibre") indicates the approximate diameter of the barrel at the muzzle - and by extension the diameter of the round fired.

Calibre is usually expressed in inches (or fractions thereof) or millimetres, depending on jurisdiction of origin. In shotguns the calibre may also be expressed as "bore" or "gauge" which is an archaic measurement of the number of spherical balls of that diameter that could be cast from a pound of lead1. Calibre may be nominal (that is, the stated calibre of a weapon may be rounded up or down a little from what appears on the machining diagrams), but someone should have already taken account of this when assigning the ammunition.

As a rule of thumb, the larger the calibre, the more powerful the weapon (for equivalent classes of weapon), although your mileage may vary considerably.

Calibre may also be used, particularly in the area of ordnance, to express barrel length - for example a 75mm 50 calibre weapon (or 75mm L502) has a barrel 75mm across at the muzzle and 3.75m long.

Early ordnance grade weapons may also have their calibre expressed in 'pounds' (e.g. the 12lb Napoleon cannon), much as for a shotgun, but in this case the shot in question will be cast iron and the 'pound' used to cast the shot inquired into very closely (as practically every nation had at least one 'pound' or equivalent measure that varied from everyone else's). This terminology continued into the mid 20th century (especially in the British Forces) but had less and less to do with calibre as ammunition became more complicated over the years.

Other systems exist - for example, Imperial Russia had its own esoteric system of measurement using the "line" (approximately 0.1"), hence the Mosin-Nagant "Three Line Rifle 1891" … which almost all modern, and many contemporary sources refer to as being chambered in 7.62mm Russian (or 0.30).


1. full source reference

Game and Story Use

  • Be aware that ammunition of the same caliber is not interchangeable - for example, the Soviet Red Army used three different types of 7.62mm bullet: 7.62x54 (aka. 7.62mm Russian), 7.62x39 (aka. 7.62mm WarPac) and 7.62x25 (aka. 7.62 Tokarev). The first was a full power rifle round, used in sniper rifles and machineguns, the second an intermediate power round, most famously used in the AK47/AKM and the third a pistol round, used, amongst other things, to feed the PPSh SMG. None of them could be substituted for the other.
    • Even the relatively similar 7.62mm Russian and the 7.62x51mm NATO equivalent could not be interchanged.
    • Likewise, the standard German rifle round of WW2 (the 7.92mm Mauser) and the round for the SG44 (the 7.92mm Mauser Kurz) fired the same bullet from very different cases.
  • Confusingly .38 special and .357 magnum can both be fired from the same weapon … although it's safer to fire the former from a weapon built for the latter than vice-versa.
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