The AR15 in 22 LR I bought was a Gen 1 DPMS 16” M-4 style upper.
I built a RRA lower with RRA commercial buffer tube and butt stock. I installed a RRA LPK with RRA NM fcg.
I use the small diameter CAR hand guard with this DPMS. I shot this upper so much I ruined the DPMS barrel. The original DPMS ten round mags did not feature a bolt hold open follower. I had dropped the hammer on an empty chamber so many times I ruined the breach end of the barrel. I used the modified DPMS barrel for a long time with a CMMG barrel and finally decided to replace the barrel with a CMMG M-4 profile 16” rimfire barrel.