To always attain the optimum spin number regardless of increase/decrease in firing speed of a shell.

In this shell, a cartridge belt 23 meshing with a rifle formed on the inner peripheral wall surface of a firing cylinder 10 is disposed in a body 21. The cartridge belt 23 is turnably fitted to the body 21, and an interlocking mechanism B is disposed to inhibit the relative rotational displacement of the cartridge belt 23 and the body 21 until the rotation of the cartridge belt 23 reaches a predetermined spin number, and allow the relative rotational displacement of the cartridge belt 23 and the body 21 when it exceeds the predetermined spin number.