if someone is doing it for you, I would make sure they measure the whole length and circumference of both tubes and identify the MINUMUM wall thickness they find. 25 or 30 thousands most spots is great unless they miss a spot where a dent had been raised and that left 15 thousands right next to your finger on the foreend.
I expect tedious to do the first few times(I'll know when I finish my set up). But someone who does it often can do it very smoothly - Jack Rowe checked out a set of barrels for me at one of the early vintagers, it would have taken me longer to calibrate the settings.
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"If there is a heaven it must have thinning aspen gold, and flighting woodcock, and a bird dog" GBE
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