The problem is that the existing barrels are NOT parallel. They start to converge right from the breech face. If you bore them parallel then you end up with walls at the front end of the barrel flats, which are noticeably thicker on the inside(near the rib) than on the outside. This greatly complicates mating the new barrels with the monoblock. I have personally found it far superior to bore them concentric to the existing chambers. It just means that you have to spread the barrels apart with your spacers but that has never caused me any problems with the four that I've built so far.