Octagon barrel is great for finding centers

[Bang]

With a little electrician's tape to protect the finish, a small vise clamped to the vertical facets creates a wall on each side of the sight, allowing for centering the sight by having each side of the sight identical as measured with a caliper. Same for finding center for drilling and tapping needs.

[Gunplummer]

Not all barrels are created equal and sometimes the bores are not central to the flats, especially with older barrels. Easy to check with a dial calipers at the muzzle.

[Bang]

True. I had written that, then decided not to include it. I was working on an H003T Henry, had checked the centering of the bore and failed to explain that. Thanks for the interest.