"Assault Rifle" is a potentially vague term, as is "Military Grade".
In the case of the AR-15, it's a civilian-legal firearm modified from what the military uses to remove the select-fire function/feature (automatic and burst fire modes).
So far as I understand, apart from those modifications it's the same firearm, so I suppose "military grade" is apt enough.
An AR-15 isn't an assault rifle, at least as that phrase is defined technically.
It doesn't have select fire, but as I already said, that's the only difference between it and an "assault rifle".
Of course, the specifics depend on who manufactured it, but I'm fairly sure select-fire (which includes automatic fire) firearms are illegal for civilian use if manufactured after 1986.
So, technically, assault weapons manufactured after 1986 have been illegal since that year, and owning older ones requires much paperwork and consequently they are relatively quite expensive and hard to get.