Can you put a mil-spec receiver extension on a RRA lower? Apparently there is a discrepancy with receiver extension diameter. The quality rifles (Colt, BCM, Daniel Defense etc.) are 1.14" dia. While the lower end rifles (RRA, Bushmaster, DPMS) use 1.17" extensions. Is this an inner dimension? Are they interchangeable or am I stuck putting a commercial extension on my lower?