Thank You
Another route may be to use a BCM upper receiver. From BCM:
"Inside Diameter for barrel extension slightly undersized for a tighter fit to the barrel and a more accurate rifle..."
I bought one of their blem/demo uppers and the diameter for the barrel extension is undersized. I had to use a heat gun on the upper and small coating of oil to get the barrel to slide in, along with a mallet. But there is no play between the barrel and receiver.
Every bcm reciever I used was tight to barrel fit. On the loose (non bcm) ones, I just stole the feeler guage from my set and slides right in to tighten loose recievers.
Thermal fit barrels. https://youtu.be/-cOlNGxUhsk
Tried using Loctite 620 with .0015 and then .001 shim stock yesterday. Difference between extension and receiver was about .005. After coating extension and inside of receiver with loctite, I definitely couldn't use .0015 shim stock. I tried .001, and just couldn't quite finish the last 1/16 of on inch into the receiver. A heat gun would have made it work very easily. I ended up using the Loctite, and throwing it together. The experience taught me what to expect, and the upper is just ratty parts to a rejected M&P Sport upper being put back together for testing muzzled devices, etc.
(We tried everything to get that upper to work, only to realize later that the BCG was to blame and S&W wouldn't replace the BCG. Having disassembled it three or four times, I just let it sit in parts for the last couple years. )
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