Actually, I disagree. "Assault" is a verb. It is something a person does. A person may commit "assault" but a firearm is just a chunk of metal and possibly polymer. It is an inert tool. It can't do anything on its own, including assaulting anyone.
The term has been applied to military type firearms as they are exclusively designed to assault the enemy. But no modern sporting rifle, or pistol, for that matter, was designed exclusively for that purpose. Even my 1911A1 is a target pistol used in competition matches. Due to modifications necessary for use in competition it would make a terrible military sidearm. It has never, is not, and hopefully will never, be used to assault anyone.