My oldest son has a Bushmaster M4 and has owned it for a year and half and since then this rifle has gone through 2 conventional upper receivers. It has suffered a broken a bolt 2 times (where the cam pin passes through) and have sheared the gas key off.
With this type of conventional upper, if the gas port gets plugged, the gun will not cycle.
Anyway when you fire a conventional M4 type upper hot gases are funneled from the barrel through the gas tube and into the gas key on the bolt carrier group. This is what causes the gun to cycle. These hot gases make the bolt and bolt carrier very hot. Heat, or more importantly the constant heating and cooling of metal causes it to fail long before it would if it was not subject to that abusive environment.
He has since went to a piston driven upper and has not had a problem since.