There's way too many other variables that are at play. We can't give you a simple "such-n-such" answer. It sound like a highly specialized gun barrel. Either a pistol, a modern sabot shooter, an Elephant gun, or a short-range target gun.
As Necchi correctly stated 1:20" twist sounds more like a pistol barrel for that caliber, OR it sounds like a modern muzzle loading rifle.
I suppose 1:20" twist could be used for very long bullets - like a .54 caliber version of a Whitworth or Volunteer rifle. But such a bullet would likely weigh in the 800 to 1,000 grain range. We're talking serious recoil or extremely heavy guns. Something like African Elephant medicine.
I would disagree with Necchi that light loads would cause skipping in a rifle-length barrel. If you had a .54 pistol that had a 1:20" pitch and it shot 50 grain charges with a roundball; there's no reason why the same load wouldn't work just as well out of a carbine or rifle length bore. In fact I had just such a gun. It was a Cabela's .54 caliber Hawken carbine, 21" barrel, 1:24" pitch. That shot patched balls really well with pistol charges (20-50 grains).
If I had to make a wild guess, I would imagine that you would cease benefitting from barrel lengths in excess of 30 inches with that twist. Just because the twist is going to limit you to small charges more appropriate for pistols. 35 grains of powder will cease exerting force on the projectile sooner than an 85 grain charge. If all you intended to do was poke holes in paper at distances less than 100 yards you could have a very mild recoiling gun. However, I would wonder why not shoot a .32 or .36 then. You'd get the same minimal recoil, a flatter trajectory, cheaper lead, & it would also be hell on squirrels.
