(1) A folding or telescoping stock,
(2) A pistol grip that protrudes conspicuously beneath the action of the weapon,
(3) A bayonet mount,
(4) A flash suppressor or threaded barrel designed to accommodate a flash suppressor, and
(5) A grenade launcher
And it also has to have a detachable magazine in addtion to 2 of the above 5.
The centerfire portion varies based on state, but it can be the dif between 2 and 3 ID rules.