SMGs of course are “submachineguns” because they’re generally full auto like other machine guns, despite firing pistol-caliber cartridges. However, my question is, why are “automatic rifles” not simply called a different form of machine gun? Surely every SMG and LMG isn’t smoothbore right? Why aren’t “assault rifles” just called “intermediate machine guns” or something of the sort when they’re fully automatic just like SMGs?
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Machine gun legally: One pull of trigger puts out more than one round (ridiculous ATF reinterpretations notwithstanding). IOW, full auto (or burst).
Now we get to the words as used to describe military weapons:
Machine gun: Full auto designed for sustained fire, often with a large magazine or belt so it can keep up that fire.
Assault rifle: Full auto using an intermediate cartridge, between pistol cartridges and normal rifle cartridges, not designed for sustained automatic fire. Meant to be the primary arm of a soldier.
LMG: Machine gun capable of being wielded by one soldier within a squad or platoon in a dismounted infantry setting. These days they usually use the same round as assault rifles because they’re lighter and have less recoil.
Medium machine gun: Same as above, but using a full-powered rifle cartridge. Usually requires two operators for dismounted infantry, especially to swap barrels because those full-power cartridges produce a lot of heat.
Heavy machine gun: Same as above, but with a much heavier round than soldiers normally use, requires a bigger crew. For example, the M2 uses a crew of four to carry it, its tripod, extra barrel, and all the very heavy ammo.
Submachine gun: Full auto using a pistol cartridge.
On fire rate, there are three kinds. Cyclic is how fast the action of the full-auto gun can fire. For the M4 Carbine this is 700-950 rounds per minute. The other is the rapid rate, how fast you can shoot, but then you should let the gun cool down. For the M4 this is 45 rounds per minute. Then there’s sustained fire rate, how fast you can shoot indefinitely without overheating. For the M4 this is only 12-15 rounds per minute. That’s why I said not designed for sustained automatic fire.
Machine guns also have real fire rates lower than their cyclic (often called slow and rapid fire), and they’re normally expressed in bursts (so many round burst, wait so many seconds, repeat). But their overall rates are usually higher than for an assault rifle, and you can swap barrels so you can keep going when it gets too hot. You shoot an M4 too much, you just need to sit and wait until it cools down.
Edit: Also, in general, don’t look for much consistency in any naming regarding guns. What’s a carbine? That definition has changed over the years, but only generally means something shorter than what’s normal at the time. A Civil War carbine is a lot longer than a modern full rifle. And then we have the names of the calibers, which are usually not exactly that caliber for various historical, marketing, and legal reasons.
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