I still own a .45 gain twist barrel and the accuracy is beyond reproach. IMHO it would not be such a bad idea for prb rifling to have just a trace of gain anyway. On most all the barrels available to us it's not needed; but nothing is worse than any amount of slowing toward the muzzle.
Bill Large made a number of gain twist round ball barrels but I can't remember the ratio and I helped make the machines.
After Bill passed away,20 years later Jim McLemore in Hebron,Indiana bought all in Bill's shop except the cobwebs and he
also made round ball barrels that really perform.Harry Pope's muzzle loaders were entirely different and used long bullets and usually started with a 1 in 16 and ended in 15 and 3/4.Pope always said the last inch of any barrel will determine the accuracy and he was right.
Early versions of the German 88MM gun that was such terror had a gain twist and Bill Large and some Army officers checked out one and
Bill told me the first time he heard it go off he knew what was going on with it.Not all 88's had this system,probably because it took longer to make and by that time Russia was a major problem for them.The 88 was originally intended and was used as an anti aircraft gun and after that it appeared on the Panzer 6-1's and 2's better known as the Tiger tank.
Bob Roller