Calibres

as described in the next chapter, the distinction between 'heavy machine guns' (HMG) and 'automatic cannon' is an arbitrary one which has varied in different countries and at different times. As far as projectiles are concerned, the generally accepted convention has been that HMGs are designed to fire ball, incendiary and AP projectiles similar in design to those used in rifle-calibre machine guns (RCMGs), while cannon usually fire HE shells or specialised AP shot. HMG projectiles are therefore usually jacketed with a relatively soft, usually copper-based, alloy which can be formed to fit the rifling grooves. Cannon projectiles are normally of steel with separate, softer driving bands to take the rifling. Two exceptions to this general rule were the 13mm Rheinmetall-Borsig MG 131 and 15mm Mauser MG 151 aircraft gun rounds, which

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