Centrifuge gun concept I've been doodling for a while. Two in-line electromagnetic centrifuge chambers accelerate a round into a linear accelerator which accelerates the round into someone's FACE.
The pistol unit can be used alone with a smaller powerpack, or the linac can be removed for a carbine. Scopes and such can be tacked on of course. The gun uses some Mass Effect style ammo system where chips are shaved off of an ammo block so one ammo pack can last for thousands of shots.