From 'A primer on cartridges' on my website:
"There is often debate about the distinctions between 'machine gun' (MG), heavy machine gun' (HMG) and automatic cannon. There are no universal rules, and naming practices have varied in different countries and at different times. However, it is now generally accepted that MGs fire rifle-calibre ammunition (5.45 – 7.92 mm), HMGs are chambered for 12.7 – 15 mm rounds and cannon for 20+ mm cartridges. As far as projectiles are concerned, the generally accepted convention has been that MGs and HMGs are designed to fire jacketed ball, incendiary and AP bullets while cannon usually fire steel HE shells or specialised AP shot. However, there have always been exceptions to this so it cannot be relied on."
Tony Williams: Military gun and ammunition
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