re: what makes an assault rifle an assault rifle?

The term assault rifle is a translation of the German word Sturmgewehr (literally "storm rifle", "storm" as in "military attack"). The name was coined by Adolf Hitler[3] as a new name for the Maschinenpistole 43,[nb 1] subsequently known as the Sturmgewehr 44, the firearm generally considered the first assault rifle that served to popularise the concept and form the basis for today's modern assault rifles.

The translation assault rifle gradually became the common term for similar firearms sharing the same technical definition as the StG 44. In a strict definition, a firearm must have at least the following characteristics to be considered an assault rifle:[4][5][6]

It must be an individual weapon with provision to fire from the shoulder (i.e. a buttstock);
It must be capable of selective fire;
It must have an intermediate-power cartridge: more power than a pistol but less than a standard rifle or battle rifle;
Its ammunition must be supplied from a detachable magazine rather than a feed-belt.
And it should at least have a firing range of 300 meters (1000 feet)

Rifles that meet most of these criteria, but not all, are technically not assault rifles despite frequently being considered as such. For example, semi-automatic-only rifles like the AR-15 (which the M16 rifle is based on) that share parts or design characteristics with assault rifles are not assault rifles, as they are not capable of switching to automatic fire and thus are not selective fire capable. Belt-fed weapons or rifles with fixed magazines are likewise not assault rifles because they do not have detachable box magazines.

The term "assault rifle" is often more loosely used for commercial or political reasons to include other types of arms, particularly arms that fall under a strict definition of the battle rifle, or semi-automatic variant of military rifles such as AR-15s.

The US Army defines assault rifles as "short, compact, selective-fire weapons that fire a cartridge intermediate in power between submachinegun and rifle cartridges.