Fast twist barrels ALSO NEED shallow rifling, to handle conical bullets well. a 1:32" ROT barrel is considered fast, but there are certainly faster ROT barrels for lead conicals.
With a shallow grooved barrel, you need to use the correct ball diameter, and the correct thickness of patch, lubed well. In my old Spanish made .45, the rifling was almost non-existent--- mere scratches in the barrel. The Bore was .451
in diameter, and grooves were not more than .002" Deep.
I found the gun shot very well using a .445" RB, with a .010" patch, lubed with either spit, or with wonderlube. I tried .440 balls, with .015, .020" thick patches, and could not get better groups-- in fact, they were worse. I was able to use 55 grains of 3Fg powder, which was a pretty hefty load in that gun with only a 25" barrel. But, out to 50 yds, It shot very well.
I have since shot .45 rifles with barrels that were 10 inches longer, using powder charges up to 90 grains. The most accurate groups, however, were in the 70-75 gr. powder charges in those longer barreled guns.
My own work, my brother's work with several .40s, and now .45 rifles, as well as talking to other shooters has convinced me that you can get accurate RB groups with a shallow rifled, fast twist barrel, and you can use large powder charges, if the barrels are long enough to burn it efficiently. But, you have to have a very tight fitting Ball/patch combination in such guns if you want to get both velocity and accuracy.
The Chunk Gun shooters are using oversized balls in their guns, -- a .454 or .457" diameter ball in their .451" diameter barrels. Obviously these are RB guns with deep rifling at is 1:48 or slower ROTs. The patching is tough. The Conicals used are getting close to groove diameter of these barrels.
