doin' da bump is entirely legal.

grasp the rifle firmly by the forearm and stiff finger the trigger, loosely supporting the rear of the firearm. keep constant forward pressure on the handguard. recoil will drive your rifle rearward and your forward pressure on the handguard will draw the rifle against your straight, stiff finger to fire another round.

see lumpy223 and order a copy of the bulletfest 2001 cd-rom. there you can watch the bump-meisters dumping mags as fast as a full auto mode rifle. the best of the bumpers are also fairly accurate.