Accuracy fixed.

[Lastrites]

I would fore go the Marine Tex as it shrinks and take a look at Devcon 10110 Plastic Steel Putty, sets up hard and will not shrink. Applying the release properly is critical though if you ever wanted to take it apart. I would almost be tempted in applying release to both the barrel ext and inside the receiver and then coat the inside of the receiver as the excess would be pushed out into the receiver in stead of it being piled up on the shoulder of the extension and potentially the threads.

Several places do sell some rather thin shim stock, would think that would be a good way to go if you don't tear it up sliding in the barrel.