Here's a gun question:

[Statistical]

8. It never was. Your question is actually reversed

Assault rifles is a military term which refers to select fire (automatic or burst fire) rifles. The term came about to distinguish it from earlier battle rifles because assault rifles were designed to fire a less powerful cartridge (and thus carry more ammo) as well as being capable of engaging targets at longer range.

The first assault rifle was a nazi weapon. It was the Sturmgewehr 44 which literally translates to "Assault Rifle 44". No the Nazi engineers weren't particularly creative in their naming of weapons.

The Sturmgewehr 44 was so radical as a concept other countries began to copy it and make similar weapons aka their own assault rifles. They called those weapons assault rifles meaning weapons which were similar to the Sturmgewehr 44. Eventually the term came to be used generically as an entire class of infantry rifles.

The AR-15 never was an assault rifle. Also "assault rifle" isn't used as a legal definition in the US. Even the NFA which regulates actual military hardware (automatic weapons, explosives, missiles, etc) doesn't use the term assault rifle. Under the NFA any weapon capable of automatic fire is a machine gun so a M-16 would be a "machine gun" although the military would never use such terminology our legal code does. There is no distinction between a M-16 (soldier rifle) and a M-2 (heavy crew served machinegun) under the NFA anything which is 1 trigger pull = more than 1 bullet is a machinegun.

Then along came the AWB (Assault Weapons Ban) which defined for the first time the word "assault weapon". Some semi-automatic rifles including the AR-15 were classified as "assault weapons". If the terminology between assault weapon and assault rifle seems similar it was intentional. Even under the AWB the AR-15 isn't legally an assault rifle it is an assault weapon.

So the real question is when did people decide an AR-15 WAS an assault rifle.

Over time the media began to just use assault rifle interchangeably with the word assault weapon. So when people use the word assault rifle today it could mean essentially anything from military hardware down to scary looking black rifle or even any semi-automatic weapon (which is most firearms in the world).