Full-auto weapons were regulated back in 1934, as part of the fight againt bootleggers and organized crime. Back then, you could buy, through mail-order, full-auto Thompson submachine guns.
The Tommy gun fired .45-caliber pistol cartridges from a 100-round magazine at a rate of about 750 rounds per minute, and it had TWO of those evil, evil pistol grips. A simplified (easier and cheaper to manufacture) version was issued to US troops during World War Two and Korea. Tom Hanks carried one in
Saving Private Ryan.
The civilian-legal versions of military service weapons were what were attacked for having certain cosmetic features. As an example...
This is a semi-automatic rifle fed from a detachable magazine (not shown) with traditional wood 'furniture'.
This is a semi-automatic rifle fed from a detachable magazine (not shown) with a black weatherproof synthetic stock and a protruding pistol grip. California and I believe Connecticut considers this an 'assault weapon', and probably New York, New Jersey, and Massachusettes as well.
This is a semi-automatic rifle fed from a detachable magazine (not shown) with a black weatherproof synthetic stock, a proturding pistol grip, and a folding buttstock. Considered an 'assault weapon' under the federal 1993 Assault Weapons Ban.
In all three cases a detachable magazine is inserted into the gun right in front of the trigger guard. Magazines are available in capacities from 5 to 90, but are typically 10 rounds in restricted states and 30 rounds in non-restricted states. That 90-rounder is expensive! And heavy.
But I digress...
The point of all the pretty pictures I just posted is this:
They are all the same gun. Not three similar guns, the
exact same gun with different 'furniture' attached to the mechanicals. A couple of minutes with a screwdriver is all it takes. Each picture is of the same Ruger Mini-14!
The Mini-14 fires the exact same ammunition as the Army's M-16 rifle, 5.56x45mm NATO. The Mini-14 and the civilian version of the M-16, called the AR-15, are identical in operation, capacity, and caliber. Both are semi-automatic rifles that shoot 5.56mm ammunition, are fed from a detachable magazine. Both can have pistol grips, laser sights, flashlights, and scopes attached to them. The Mini-14's design is considered somewhat more reliable, the AR-15's is considered more accurate.
Nicely-equipped, high-quality AR-15:
Mini-14 with upgrades so it's similar to above: