Upper Receiver????

[Old Bob]

Dont know, never measured any of my barrel extensions.

I check to see if it will go in by hand, if it does, I get a piece of .003 thickness steel shim stock and check it. If it still goes in I get a peice of .005 shim stock.

Never used a peice thicker then that.

I then get the shim trimmed how I like and then freeze the barrel for about 2 hours and then heat the upper, then install it and leave it alone too return to room temperature. Then I install the barrel and barrel nut.

The barrel I referred to above is an 18" Odin 6.5 Grendel barrel but I'm probably not gonna use it. What I'm most likely gonna use is an 18" Wilson Combat in 223 Wylde (.998" extension diameter) that doesn't have muzzle threads. I stumbled into a large pivot pin hole (.315") Colt (I think) upper with an inside diameter of .999" (looks virgin, never used). I already had the large hole lower receiver (since 1994). Seeing how I already have a 223 Wylde AR, what I was thinking of doing is installing a CMMG .22LR converter into it & use it as a practice plinker. Being the difference in the receiver bore & the extension is only .001", I'll start with the .001" shim stock. That should still get me a tight fit between the two components.