When my son was at the MCRD, he wrote me that he missed his Mauser (chambered 7.62mm) and said the M4 felt like a tinker toy. The 5.56mm is proving less effective in the open country of Afghanistan than it did in the jungle in SE Asia. I carried an M14 and my dad had a BAR in the South Pacific; both full auto capable and superior to the M4-IMO.
The difference between a 14.5 inch and a 20 inch barrel.