I think most of the work done on such guns is handled in small gun shops and even by individuals themselves. My best friend Billy Doss (now deceased) got into that very heavily many years ago and dragged me along sometimes tho never got me involved seriously. He got to making his own.
What I can tell you for sure is that the barrels are generally always back bored and forcing cones are lengthened. Chokes are more art than science and are key to winning. Some times a barrel straight from the factory will work wonders and sometimes it has to be worked over big time. Different places shoot different shot sizes and each shot size requires a bit different gun. Where Billy shot they used #8, # 9 and #10 shot. All the shooters had different guns for each. No one shot the same gun for more than one shot size.
I watched and some of the secrets of the winners seemed to be cleaning the barrel with a bore brush after every shot. No one who was a consistent winner failed to do this. Dunno why but they felt it mattered and those who didn't do it didn't win. Atmospheric conditions sure made a different in the way barrels shot. Some did best on damp and cool days, others were best in hot dry conditions. The best and most frequent winners had lots of barrels or guns for the various conditions.
Putting the pattern assuming you have a really tight one in the "right" place is important. For that you need a good sighting method. No one won who used the beads on the barrel. Everyone used either scopes or peep sights. Also no one I don't think sighted in dead on. This was "just in case" somone insisted on being allowed to use the same gun which I think was a rule they must have had as it seemed anyone could use the gun of another to shoot by asking.
Those #10 guns shooting 2 oz. ate holes big enough to put a coffee cup thru in the target. Problem was it seldom seemed to be in the center of the target.
Stan Baker does guns for this specilized game or at least used to. I'd start with him. That's the only name I can help you with as it has been many years since I last attended these kinda shoots.
GB
