How do you bed a barrel to the upper receiver?

[LRRPF52]

We've beat this subject to death over the years. Starting with a square receiver face as a premise....here are the various methods you can use to make a more secure mating between the upper receiver and the barrel extension:

* Thermo-fit
* Shim stock and adhesive bedding
* Adhesive bedding
* Press-fit with matched tunnel ID to extension OD
* Metal epoxy build-up on extension, followed by turning to specific OD on a lathe

* Balanced torque specs with any of the above (harmonically-balanced torque specs depend on barrel mass, with lighter barrels getting more torque, and heavier barrels getting less according to extensive testing by AMU, talking ~40-65ft-lb ranges)

Then comes muzzle device balanced torque, and optic mount/rings balanced torque.

This is all under the assumption that we want a highly accurate, precise rifle/mount/optic combination. Each method has its advantages and disadvantages, with Thermo-Fit being pretty ideal, but requires very tight controls on surface thickness of whatever process is used to coat the aluminum upper receiver. Hard coat anodizing is very difficult to control thickness levels to that dimensional requirement for a manufacturer. There's a solution to that, albeit a very expensive one.

For the home-builder, you can buy several parts and hand-fit to find the ones that need thermo-fitting, or see if parts suppliers will provide you with minimum ID uppers with maximum OD barrel extensions, or just use the adhesive or adhesive with shim stock method.

By using the tight-fit parts and adhesive method on a square receiver face, I'm getting 8"x4" 6rd groups with a Wasp contour Lilja 18" barrel at 1000yds, and rapid-fired mag dumps into the heads of 18" silhouettes at 600yds on video.