The M-21 trigger is mechanical.However, the inertia weight that prevents doubling is sensitive to recoil.If it does not swing forward, doubling occurs.The Miller has the same problem,if the weight does not move to the rear. All of them are very critical as to the relationship of the parts. The older tang type guns especially so. However, a K or P gun can give fits. THe K being mechanical and the P usually inertia, but one of mine , used by the USA team in the L.A. Olympics, can be set mechanical(factory).