Some time back I started a project with a slow twist barrel for round ball and full diameter slug in 20ga.
Had Bauska barrels cut a 26" .625 with a 1/70 twist.
Nice barrel but I couldn't make it shoot beyond 50 yards with standard slugs -2 3/4 Brenneke and BRI. At that range it grouped within an inch but at 100 it was closer to 6" and worse with the BRI style.
I was under the impression that it had to do with the combination of the attached wad and going subsonic.
Brenneke also makes a 3" that stays supersonic to 100. I opened the chamber with an extended cone reamer and eureka - 2.5" (in the wind no less)
I really can't think of any other reasons for the accuracy to improve that greatly. Just an addition for those looking into projects like this one.
Tom
