Why do SXS barrel bore diamiters vary?

[Cold Spring]

I agree that generally each bore should be within say .003" diameter of the other. But keep in mind that most shotgun makers bought rough tubes to their individual specs from suppliers, and an overrun for one maker could be a bargain buy for another maker. And thus, tubes with different bore diameters could find their way to the storage racks of some of the makers. Generally the barrel assembler would choose two tubes of like bore diameter for a barrel set but that may not have happened all the time. For example: an Ithaca 16 gauge on my bench now that has right tube at .685" bore diameter and choke .671 = .014" constriction. Left tube .674 bore diameter and choke .646 = .028" constriction. Gun is stamped 2 & 4 indicating Mod & Full and the choke diameters are spot on. This gun is absolutely "factory".

For those not familiar with barrel honing (not backboring) it can only be done UNIFORMLY using a Sunnen barrel hone or equivalent machine tool, and they are very expensive and not usually found in the shops of gunsmiths. Most gunsmiths used/use simplistic tools to "hone barrels" - such as emery cloth on the end of a mandrel spun by a drill motor or maybe a lathe. This does not produce UNIFORM bores. Therefore if you're reading larger-smaller-larger (variable bore) with your barrel mike, it's probably been "honed" by some gunsmith using emery cloth or similar. But if your mike is showing one bore is larger than the other, but it's UNIFORM to within .001" diameter along its entire run, that barrel is likely factory. The other possibility is it's been backbored using a lathe and reamer(s) but in that case a close visual inspection will show a difference in surface finish/appearance from one bore to the other.

Just my take on it fwiw.