my only prob with my M4 is that the action is a little slow on light loads(which can be expected) i can shoot faster than it can cycle, which is a problem im running into more and more when im workin on 3+ round strings.
one major plus is, my m4 has cycled on bags and batons. which i wasnt expecting.
i love mine, though i must say, for speed shooting, i miss my m1... but i would never get rid of my m4, ever.