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We don't have a procedure for it, as we don't put comps on our varmint barrels as a general operation.
The barrel would have to be turned, probably in two or three operations, from the bull profile to the profile that you want to have it in when you're done (generally just about the same size as the comp), possible with an intermediate step to make the transition look "normal". Then the muzzle would have to be turned and threaded for the comp you want to install. You would probably have to match the profile of an existing barrel to get the "shelf" rearward of the thread and forward o the main barrel diameter. At that point, assuming you want the most accurate barrel you can get with a muzzle device on it, I would probably cryo the barrel.