Assuming the tang sight is mounted correctly, the limiting factor for accuracy will be: How repeatable is assembly of hook breech-to-tang?
This is analogous to a Q-D scope mount. There must be slight slop in the assembly or it would not assemble. I just bought a "shooter" Lyman GPR. I believe the tolerance question is moot. Powder fouling will almost certainly conceal any minor seating of the barrel assembly.
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I have a question for you, though. Since the Lyman rifles, especially the GPR, are not bad looking, have you considered investing a little more money to buy XS Systems' Ghostring Aperture sight system? Even though I intend to use my GPR for a short time, I'm leaning toward XS sights because they do not detract from the overall appearance of the rifle nearly as much as Lyman's. . . . Not a recommendation, just a thought.