Gains for bullets with long bearing surfaces need to be slight, like 1/2" gain or the bullet will have "skid" marks as the front of the bearing surface is in a faster twist than the rear. If the bullet is like a cloth patched "picket" bullet with a very short bearing length then far more aggressive gains can be used. Such as a beginning of 60" and ending at 30".
I would also point out that for long bullets (3 calibers plus) in small arms at BP velocities Greenhill is off about 2". IE if it calls for 20" then 18 is probably the better choice. The formula was designed as a tool in determining the twist for rifled ML field pieces not small arms. 3 caliber long slugs of this type are not used except in slug guns or rifles like the Whitworth that shoot bullets that were the precursors of modern high sectional density bullets.
For a RB I am not sure the gain twist is an advantage but I am not sure inf NOT either.... What I DO think is that the gain twist came about for perhaps one of two reasons. To prevent stripping the patch during initial acceleration when the picket bullet was adopted and it was found that is shot better with a twist faster than the typical 48" twist. It also shot a lot more powder than a RB in most guns. When I stopped playing with it my 48" twist 40 cal barrel was us to 80 grains of FF Swiss. It may have been a rifling form issue as well. When I was a kid I had a I had a restocked barrel that was turned for a starter. At the time I was too ignorant to know why. It had a 7 sided bore with grooves at the corners and I could not really see how this sealed well. I shot the barrel a little but apparently could not make it shoot. Don't remember. Its been 50 years. It could easily have been a gain twist. But I did not know enough to check
The second possibility is a rifling guide with an inconsistent twist rate that put a slower twist at the muzzle. This is really bad with bullets and can't be good with a RB. Making a guide with a positive gain would eliminate this and the gain twist would be far more accurate. If the uniform twist guide was truly uniform it would shoot as well as the gain, probably. This said there are "modern", top flight barrel makers that will make a (very slight) gain twist barrel that is extremely accurate. They will also make a uniform twist barrel that is extremely accurate.
My most accurate ML has a McLemore gain twist barrel. He will not give the twist rate. But is so accurate that it would require a good scope to make full use of it. But this is a combination of barrel weight (17 pound 12 ounce rifle), a uniform piece of high grade barrel steel and the extreme CARE he put into drilling, reaming and rifling AND testing the barrel to be SURE it was up to his standard. The slug gun makers of the 19th c. made some fantastically accurate rifles (though very heavy as are most super accurate "moderns"). Until the 20th C the brass suppository guns simply could not match them in 40 rod competition. This was done by their attention to detail. This is true today.
I think experimenting with a gain is a great idea. Other than making a guide with a uniform gain I see not downside. But like most of what we do we are reinventing and rediscovering the past. Which is really what we do all the time. If I intended to shoot a traditional 2 caliber long picket I would try for an ending twist in the 30-36" range. If the caliber is 50 or over then 40-48 might be OK but if rifling a smaller bore it might be too slow. For RBs the twist could end at 40-48 for any caliber.
Dan
