maybe I should clarify something based on your frustrations. I do think there is silliness written in the rules, but in the example you mentioned, it does not make the gun illegal, just not production. Likewise with a better barrel. Unless I read the rules wrong.
Once again, if you based classes on the three things I suggested, you would have the following big-bore classes:
Standing, open sight, full size (S)
Standing, open sight, half scale (SHS)
Standing, any sight, full size (SAS)
Standing, any sight, half scale (SASHS)
Any position, open sight, full size (U)
Any position, open sight, half scale (UHS)
Any position, any sight, full size (UAS)
Any position, any sight, half scale (UASHS)
your frustrations about piddly rules and big money guns vs home-engineered guns go away. No need for a mile long list of acceptable production guns either. In fact, I really don't think that some of the talent I've seen needs to spend a fortune to be good. They simply are good shots.