Barrel makers may be their own victims of what the foundry sends to them. Usually, Inclusions in the barrel can not be seen. Dead spots are often found in a barrel that will give strange harmonics.
To detect dead spots, tap a barrel with a 5/8"-3/4" piece of round brass rod. The sound will start at tink, tink, tink, tink. When you hit a dead spot, the sound will go tink, tink, thunk, thunk, then back to tink, tink. Do not cut the barrel off within 2" of the thunk sound, or your accuracy is screwed. You can try this on barrels that you have had that were difficult to tune. An old German Gunsmith taught me this, passed on to him by some old European Master craftsmen.