Semi-automatic rifles able to accept detachable magazines and two or more of the following:
Folding or telescoping stock
-Pistol grip
-Bayonet mount
-Flash suppressor, or threaded barrel designed to accommodate one
-Grenade launcher (more precisely, a muzzle device that enables launching or firing rifle grenades, though this applies only to muzzle mounted grenade launchers and not those mounted externally).
These are pretty ridiculous IMO, but whatever... anyway...
Take an AR-10 for instance... Couldn't a manufacturer simply alter the current design to remove some of these features?
Put on a Fixed stock, unthreaded barrel and a fixed rail with no railing on the bottom (to prevent the addition of a pistol grip or grenade launcher)