The uppers receive a detail cleaning every 4,500 rounds. Two of the barrels (out of maybe 20? uppers) had ruptured at the portion midway underneath the handguards. One before its first cleaning and one at around 6,000 rounds (just in case you were curious how long an M4 barrel can survive an extreme firing schedule). The people doing the testing indicated such ruptures were not unusual and at least one of the test uppers had been replaced with a thick HBAR style barrel (no M203 cuts). Since few of us get the resources to test uppers in such a manner, I thought folks here might appreciate an idea of what an M4 barrel is capable of handling.