In 2007 Dust Test, ten rifles of eeach kind, i.e. 10 M4s, 10 SCARs, and 10 HK416, fired 6000 rounds each, a total of 60000 rounds per rifle.
The M4 stopped 882 times, and SCAR stopped 226 times, and HK416 stopped 233 times. So if SCAR is a nightmare, the M4 is a much worse nightmare.