I think a pistol grip and detachable magazine are two traits of "assault weapons." But practially, if it looks like something the military uses, then its an assault weapon.
Functionally, there is no difference between my AR and my marlin .22LR. Both are semi-automatic rifles.
Not trying to be an as*, but then couldn’t you classify a pistol as an assault weapon? My Walther P22 has a pistol grip and a detachable magazine with 10 rounds of 22lr. Does that make it an assault weapon? I agree with what bubbat said, that basically anything that shoots a projectile out of a barrel at a high enough velocity to cause serious injury or death could be technically used to attack or “assault” a position or person or group of people in a fortified position.
The media and the government like to throw around the term “assault weapon” or “assault rifle” to scare people who know nothing about firearms. “Assault” rifle sounds scary and to someone who is untrained or unfamiliar with firearms will believe anything that the media or government tells them an assault rifle is.
Also, do you consider the Remington 700 to be an assault rifle? Military snipers use the Remington 700, as well as hunters in the backwoods of Mississippi. Does that mean since the military uses that rifle it should be considered an assault weapon? (Of course that answer depends if you are the hunter or the deer on the other side of the scope.)
The Military channel did a show that had a countdown of the top 10 assault rifles in history. Some of the weapon systems that made the list was the 1903 Springfield (a bolt-action 5 round capacity rifle), the Mauser K98k Carbine (a bolt-action 5 round capacity rifle), the M1 Garand (semi-auto yes, but I don’t know if you can consider the 8-round clip as a detachable magazine), and the Lee-Enfield SMLE (bolt-action 10 round capacity rifle). So technically would you consider those to be assault rifles?
Just throwing my .02cents out there. Like I said, I am not trying to come across as an as*, just giving my perspective on the issue.