What is an assault rifle?
First it is a weapon that fires projectiles, doesn't necessarily have to be bullets, in the future it could be high energy plasma etc. It must be of a make that has rifle characteristics eg long barreled. There would be similar weapons in military use but those are obviously full auto and the civilian versions semi auto. The fact that the military uses such a weapon type whilst not necessarily the same model is key in identifying if it in fact is an assault rifle. Some may say you can upgrade a hunting rifle to be used in a similar fashion and maybe you can but is it in military use?
Another important characteristic would be a large capacity magazine. A assault rifle with a 1 round mag would almost instantly lose any effective defence or offence capabilities for the user dereby making it lose the assault part of the name, you cannot do much assaulting with 1 bullet. It would also need to be a weapon that uses rifle rounds but not necessarily exclusively. I have seen ar15 variants that use 9mm etc. I am not sure if they are still what a rifle is intended to be.
I am not anti gun but I just played with the question being asked. Debate me.
Extended mags have nothing to do with a gun being a “assault” rifle. Extended mags are made for a variety of guns like bolt action rifles and semi- auto handguns. Biggest difference between a military used M-16/M-4 and a civilian ARMALITE RIFLE (AR) is that the military rifle is select fire. Which means that you can choose between shooting 1 bullet per trigger pull or 3 rounds per trigger pull also called burst fire. The civilian rifle ONLY fires 1 bullet per trigger pull or semi- automatic. I realize their are military models that also fire full auto but they are rarely given to Ave. GI Joe.