Even if the outside barrel contour doesn't have a taper, reaming is going to reduce wall thickness. So will honing to take out pits. You remove metal, you have less metal left. The question is, how much did you have to start with, and how much is left when you're done. Lengthening chambers only 1/4" (as on Brit/European guns with original 2 1/2" chambers) can be perfectly safe; witness all of those to which it's been done, and which have passed reproof. Most of the time, however, you'll get the same benefit, with far less metal removed, by simply lengthening the forcing cone.
