Reasons for an absence of markings on a bullet:
(1) fired through a barrel without rifling
(1a) smooth bore gun
(1b) hand-made gun ("zip" gun)
(2) fired through a revolver with the barrel removed
(3) sympathetic discharge of a rimfire bullet in a revolver
(4) sabot round (bullet fired with a casing that falls off after the bullet leaves the barrel)
Most of these conditions require gun discharge close to the victim:
(1) A zip gun cannot handle high pressure, so must fire a small bullet.
(2) A bullet fired from a smooth barrel or with no barrel quickly becomes erratic in flight.
(3) Sympathetic firing requires a rimfire cartridge, which implies a 0.22 caliber round. While sympathetic firing may prevent rifling marks, it often leaves marks where the bullet hits the frame of the gun.
The sabot round is the only one of these methods which allows a high powered bullet to hit a target some distance away with any certainty.
