Bores may not be centered in the barrel. Your assumption that the .066, .800-.734=.066, is evenly divided in all directions, is not certain. So your .066 may be .033 of both sides and could be .044 in one direction and .022 in another. Worse when you have a gun which has had a dent removed and the barrels filed down and refinished you will have a thin area. Not trying to confuse you or alarm you. But we measure wall thickness not internal and external dimensions and do a little math. And measuring is as much an art as a skill. Good equipment, poorly used, will not give you true results.
