PURPOSE: To achieve higher hitting accuracy with the turning of an additional barrel by mounting the additional barrel with a spiral groove worked on a barrel of a shot gun with a lock nut through a bearing.

CONSTITUTION: An additional barrel 1 is provided with a spiral groove 1a, a bearing 3 is mounted on a slide surface between the additional barrel 1 and a lock nut 2 and the additional barrel 1 is mounted firmly at the tip of the barrel 4 of a shot gun with the lock nut 2. The moment a resin wad reaches the additional barrel 1, a combustion high pressure gas of powder flows intensely through a gas introduction port 1b and the spiral groove 1a and discharged through a gap between the lock nut 2 and the additional barrel 1 to rotate the additional barrel 1. Thus, a shell head or the like travels being twisted to generate a gyro effect of the shell head thereby improving a striking force and hitting accuracy.