The strict definition of an "assault rifle" is selective fire, full automatic or three-round burst.
The phrase "assault weapon" was coined by the totalitarians to fit their agenda. It has no meaning apart from what suits their agenda. To their mind, stock, magazine, forend etc make it a military weapon. Wrong. It's the auto firing mechanism. But they neither knew nor cared. And that's part of the problem.
A semi-auto rifle (one bullet fired per trigger squeeze) is NOT a military weapon no matter how badly its appearance frightens the quiche eaters.
A .223 Bushmaster is a "powerful" weapon only compared to a BB gun. It is based on the M16, whose cartridge is designed to wound rather than to kill so that opposing troops would be taken out of the fight to care for the wounded.
If you want a "powerful" rifle google the .338. Or for that matter any of the hunting cartridges designed to kill elephants, cape buffalo, polar bears etc.
Again, the gun grabbers twist the language to suit their totalitarian agenda.
Magazine size is irrelevant. A rifleman who practices can change any size magazine in seconds.
Someone who may have no skill whatsoever is not a rifleman simply because he shoots one.
two references for those who want to speak from evidence rather than emotion
http://kontradictions.wordpress.com/2012/08/09/why-not-renew-the-assault-weapons-ban-well-ill-tell-you
"In Defense of the Second Amendment" by Gary North.
http://www.garynorth.com/public
and the definitive resource:
The Founders' Second Amendment origins of the Right to Bear Arms.
Stephen P Halbrook, Esq.