Easiest fix is replace the direct gas impingement system with a gas-piston system (a la the AK). POF has a nice implementation of this, and make sure that the standard M855 ammo is replaced with the Mk262 mod1 77gr 5.56 ammo. This would increase both the reliability and the lethality of the system while maintaining almost 100% of training and some 80% of parts.
Other than that, we could go for something exotic, like the SCAR system or just wait for the Phased Plasma Rifle in the 40-watt range.