Why dont you look at getting an MRP upper from LMT? Their system is top notch with a one piece carrier and gas piston rod as well. Whats more, the versitility built into the system for barrel changes is a nice option. However this versatility comes at a price, the barrels are proprietary. I'm just buying two more barrels over time (an SBR and a 6.8) and I'll be set.