Before you say buy a new upper, I have a HK MR556 that I had converted to a HK416 CAG clone. I went with tan anodizing and I notice the fit isn't as tight afterwards. It appears to me the rear upper receiver take down hole is the culprit as it is allowing the upper to slide forward 1-2mm. I've tried the Accuwedge which doesn't help to suck the 2 receiver halves together. Oversized pin doesn't work because it is too big for the lower (those tolerances seem to be within spec), and still not big enough for the rear upper hole.
I'm thinking the best way would be to install a bushing by a talented gunsmith who can line it up realign as necessary to make it a tight fit, but that's just my thoughts. Any other suggestions or recommendations on who could do this without messing it up or has experience with this. I love the gun otherwise and want it to be perfect. Thanks!