LL will work in both 5.56 lower & the 7.62 variety. RDIAS only fir the smaller sized lowers.

I'm very happy with the way my register M16A2 works. Got to shoot it over last weekend. Both the Colt CQB upper & my LMT piston upper ran like scaled dawgs! Ran my Quicksilver suppressor on the LMT without any problems as well.