I have been unable to find a satisfactory answer to what the difference in characteristics is. I know the 21 buffer is twice as long and appears to be single stage instead of 2 stage? Would it be safe/desirable to put a 21 buffer in a 91 style rifle, or a g3 heavy buffer in an hk21? I've acquired an hk21 buffer, is it ok to put it on my 91 clone? It fits fine in the JLD stock (after opening up the diameter of the hole in the steel plate), just don't know if I'd be hurting anything by using it.