pretty much anything not a receiver, frame, barrel, or "assault weapon feature" is an accessory. while it's not likely to be enforced against screws, pins, springs, sights/scopes, tools - it was largely discussed as pertaining to parts that make functional firearms from kits or repair/replacement parts for the same (gas tube not so much, but a barrel or pistol/vertical grip sure as those are explicitly called out). various efforts to dove tail this into 3d printed receivers (so parts kits containing mentioned parts/accessories) will also be affected as well as magazine parts, or even compliance parts.