120,
The bolt riding forward on the M3/M3A1 always kept me from getting good accuracy out of it.
That's a matter of practice (me thinks). She was never designed around ranges beyond 100 meters. But then, the M4 is considered to be effective to 150 due mostly to its short barrel. The tanker must exit the vehicle and commence firing. You can do that with an M2HB (my preference) nor M60.

JC, Yes, the M4 is easy to use and maintain. But if you started out like I did when the M16A1 was a total failure, it was hard to beleive the M4 would far better. Granted, it was rarely slung over one's shoulder for any length of time, so barrel warp would be insignificant.

The H&K is a nice and expensive toy and I doubt we will get these anytime soon.