I pulled out some literature I had on the XM8. Turns out that HK did prepare a special set of quad rail XM8's for presentation as a SCAR candidate. They were prepared to meet full SCAR specs. They were turned down for submission because SOCOM changed their minds and decided not to accept Government to Government submissions.
Also the XM8 was a tested in the ATEC Extreme Dust Test (EDT) They tested 10 sample guns from 4 manufacturers. Rifles Tested were the HK XM8, FN SCAR, HK416, and the Colt M4. Each rifle was subjected to frequent "dusting" and cleanings for a total of 6000 rounds each.
The results:
HK XM8 - 127 Stoppages/Malfunctions
FN SCAR - 226 Stoppages/Malfunctions
HK416 -223 Stoppages/Malfunctions (interesting to note here was that 117 of the total HK 416 stoppages was from one weapon).
Colt M4 - 882 Stoppages/Malfunctions
I'd hazard a guess that the XM8 is probably a better weapon system than it gets credit for...