Not just not concentric, but the inner bore, and outer wall will not be precision circles in cross section. We are talking mostly hand made guns here, the barrels hand filed and fitted by a barrel maker, who may or may not be a master of his art. Then factor in older guns, which may have been subjected to barrel repairs, bore honing etc. that may or may not have been carried to by a qualified, and competent smith. There are a myriad of conditions that will lead to localized wall thickness variation.
The current tools used to measure wall thickness aren't perfect, but are very affordable, and with patience and skill can give a good indication of wall thickness. But it's always prudent to have a competent SXS specialist gunsmith confirm your findings!
