I've got an RRA lower on my custom M4,it's great,absolutely no problems.Nice finish as well,looks great.Rock River Arms is one of the best "small" companies out there,or I should say,simply one of the best companies period.Anybody on this board who has any experience with them,in terms of service or products,will tell ya they rock the house.They sent me a FREE A2 stock,simply because my lower was going to be a day late for delivery.Can you believe that? love em.