Ok, but my Hastings 12 ga 34" twist barrel groups are much smaller than 2.5" at 100 yards with Brenneke's (gold or green lightning). Modern shotgun barrels that are made for lightfields are 34" twist. The paradox choke on my vintage W. Evans is 36" twist. Seems to be a pattern here for rifled shotgun barrels firing full bore slugs 1000-1250 fps.

My 16 bore ML patched ball rifle is 66" twist and that works well. I have never tried a Fosbery (or Brenneke) shaped slug in that though.

I would like to know the twist on other vintage ball & shotguns, ie. Lancaster Colindia and all the "invisible" rifled versions that were made for elongated slugs at similar velocities.

Thanks for the info on rifling costs, sounds very interesting. You have some great projects there.