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!

Perhaps tighten the fit with light coats of diluted clear nail polish?

Congrats you are finding out what mil-spec is.

The upper lower fit that are tight are not mil-spec. Manufacturers change the spec for those consumers that think they must be tight. You check Colts when they use to be mil-spec all their fit is lose, hence the accu wedge invention.

I set up a manufacturing line for a customer and had to explain this very thing to them. They also had me change the dimension to have a tight fit but I warned them that there could be certain upper lower combinations with other companies that would not fit w/o a hammer.

You say you "went with tan anodizing", does that mean it was refinished? Someone stripped / sandblasted your very expensive parts which are now completely out of dimensional specs from the sound of it.

This!

Try an oring up front. The bushing will work but it would need drilling out the hole. Me personally I'd live with it and use an oring.

Yes that is correct. It was stripped and anodized.  I've had many parts chemically stripped and reanodized before without any noticeable changes in tolerances even though there is some material removed of course, but this seems is a bit looser. I can live with it yes, but if there was a relatively easy fix I'd be down to try it. I'll try and post up some pictures to show the fit to give a better idea.  Appreciate the feedback.

Yeah I won't try to lose sleep over it for sure.  I would say now it is probably similar in fit to the prior Colt mil-spec as you described.  I checked my current Colt's and those are all pretty snug and tight surprisingly.

The rear lug on an upper is hot-dog shaped front-to-back.  Only the front hole is round.  That's per the mil-spec print.

Go look at the rear lug hole in your upper.  It's not a round hole.

If the recoil forces hit the rear lug, you will break it off.  Maybe not with a 5.56, but with the larger calibers it will.

Tony