OK let me help you here.
Lets assume that one of the M4s issued to one of your soldiers is really inaccurate. The first thing that will become apparent is that the soldier will appear to be unable to shoot at all accurately. So what are your options?
Same instructor/coach for a bunch of guys. So the instructor takes the weapon and shoots a few five round groups himself. He knows his ability and if it proves to be the weapon you arrange a swap out from the armoury. If it is not the weapon you get back to dealing with the soldier and coach him to improve.
The instructors test is the fall back position to confirm whether its the weapon or the soldier.
Now that was not so hard was it?