That info is actually flawed, the Hk416 had one problem gun which skewed the results - if the "lemon" was discarded as an aberant fluke - the Hk416 actually scored better than the XM-8 which "won".
Aberdeen Extreme Dust Test I = M4 vs M16A4 test in 2006
Aberdeen Extreme Dust Test II = M4 test summer of 2007
Aberdeen Extreme Dust Test III = M4 vs MK16 vs HK416 vs XM8 test November 2007
Carbines/rifles were subjected to 25 hours of constant, heavy dusting in laboratory conditions at Aberdeen Proving Ground, Md., 10 of each weapon fired 6,000 rounds apiece. They were fired in 50 120-round cycles. Each was then wiped and re-lubricated at the 600 round mark. After 1,200 rounds were fired from each weapon, they were fully cleaned and re-lubricated. All weapons exhibited significant wear that rendered them unsafe for firing beyond 6,000 rounds without replacement of the barrel and/or bolt.
MRBS (Mean Rounds Between Stoppage) for Class 1, 2, and 3 combined. A class 1 stoppage is one a Soldier can clear within 10 seconds; a class 2 stoppage is one a Soldier can clear, but requires more than 10 seconds; and, class 3 is a stoppage that requires an armorer to clear:
XM8: 472.5 MRBS (Extreme Dust Test III)
MK16 SCAR-L: 265.5 MRBS (Extreme Dust Test III)
416: 257.5 MRBS (Extreme Dust Test III)
M4: 195.5 MRBS (Extreme Dust Test II)
M16A4: 118 MRBS (Extreme Dust Test I)
M4: 89 MRBS (Extreme Dust Test I)
M4: 68 MRBS (Extreme Dust Test III)
M16A4: 28 MRBS (Extreme Dust Test I with light lube)
M4: 6 MRBS (Extreme Dust Test I with light lube)
MRBF data, this is a Class 3, meaning mean rounds before it is inoperatable (I can only find partial data):
XM8 5,454.5 MRBF (Extreme Dust Test III)
M4 5,454.5 MRBF (Extreme Dust Test II)
M4 3,158 MRBF (Extreme Dust Test III)