The terms 'assault rifle' and 'assault weapon' are not interchangeable. 'Assault rifle' has a specific military term; the most notable characteristic of the assault rifle is the selector switch, that allows the wielder to select between fully automatic, three-round burst, and semi-automatic fire. A semi-automatic AR-15 is therefore, strictly speaking, not an assault rifle. 'Assault weapon' has no specific term, so people will generally use it however they like. Pro-gun control people tend to use it to refer to any weapon they don't like, and pro-gun rights people tend to point out how if taken super-literally, it can be used to describe a rock or a stick, and it's therefore a silly term. And then a bunch of other terms such as 'machine gun,' 'long arm,' 'sporting rifle,' and so on show up, and the whole arena gets muddied up so no one knows what they're talking about - which is kind of normal in modern politics. I try to encourage people to use 'semi-automatic rifle' when they're talking about semi-automatic rifles. Sometimes they listen.