I have a 20ga. 28" Miroku Daly Superior grade that ended up with a backbored set of bbls to remove the tool marks made by a previous owners attempt at 'something' which was never really all that clear to me. I guess he felt the bores were too tight and ran something like a drill bit on an extension down them toward the chokes to relieve them! The forcing cones are a bit longer now than they were originally. I simply had the bores cleaned up & polished. Chokes were fortunately not removed and I had them cleaned to up to be roughly .020 & .025 as I recall. I have not measured them for this reply. FWIW, that gun is still a bit muzzle heavy. The bores are not the same dia., each only got what was necessary to remove the tool marks & clean them up. The work was not done w/any intention of improving things, rather it was a salvage op on a very inexpensive acquisition.
As far as magic goes, there isn't any. You could expect a bit lower pressure [& velocity] from the bigger bores, all things being equal. Is it detectable? Probably, with gauges, but not by me shooting it. The gun throws good tight patterns with several dif loads using plastic wads. It did not/does not obturate consistently with card, felt, or Lujtic Mono-wads .. just got curious and I tried all three at the time.
Anymore, when asked a question as yours poses, I'm more inclined to say that the same effort/money spent on lessons with a well qualified coach, and shooting frequently, provides much better returns and results that are much more easily measurable.
The bbl. work mentioned above was either done by Kirk Merrington when he first came over & was working for Billy Hodge or by Scott Mayes who is currently the resident gunsmith for Beretta Gallery-Dallas, but it was in the 70's & to be honest, I just don't recall which of the two did it or if I had one do the bbls & the other the chokes. I would not hesitate to check w/Mike Orlen for that type of work today.
