PURPOSE: To provide an attachment capable of making the diameter of a magazine having a small diameter size as large as that of a magazine having an ordinary diameter size.

CONSTITUTION: A C-shaped spring 7 is incorporated in a magazine cover 1 having a curved part 2 which is curved in C shape and a projected lug part 3 shortly overhung from one side of the curved part 2. The curved part 2 is provided with a recessed groove 4 for engaging a chuck and a recessed groove 5 for a spring on its outer peripheral surfaces near both ends in an axial direction. The recessed groove 5 passes through a through-hole 6 bored in the vicinity of a boundary between the curved part 2 and the lug part 3 and the spring T is fitted in the recessed groove 5 through the through-hole 6 or a slit.