Just re-read my previous post see its "poorly written".
The problem with the uniform twist is that it needs to be uniform. It is all too easy even with modern rifling techniques to end up with a twist that increases and decreases down the length of the bore.
As Daryl points out where a barrel is cut can make or break it due to variations in twists and bore dimensions or harmonics.
The gain twist assures that the projectile has a positive increase in rotation to the muzzle.
A guide that has a reverse gain can be very bad for accuracy even if its only for 1" when its at the muzzle.
I am not sure this is all that critical for the RB but with the picket bullet and slugs, accuracy at long range became important. Rifles were now shot a 220 to 880 yards and this is a far different game than accuracy at 100 or 150. A barrel with a slight decrease is twist at the muzzle is not likely to shoot well with a bullet that maybe marginally stabilized to begin with.
So a maker with a guide that was supposed to give a 48" uniform twist that produces poor accuracy due to some then undetectable reverse gain or a change in twist near the muzzle could produce far better results if he made a gain twist guide.
It is possible with modern technology to measure the twist of the barrel and thus know where the problem spots are, if they exist. Thus it can be cut at the right spot to assure that the twist is not decreasing at the point where the bullet exits the muzzle.
Bullet shooters, picket or slug gun, must be far more critical of twists and rifling depth etc etc. The RB shooter can use almost any twist from about 40" to 100" and get good results with most calibers commonly in use. The most common is 48 to 72". A 2 ft variation of twist that will work in a wide variety of calibers with the PRB.
In bullet guns 2" variation in twist can make a major difference in accuracy, good or bad. The longer the bullet the more critical it gets. A short cloth patched picket bullet in 40-45 caliber may work fairly well with a twist from 30 to 48" though 30 *should* be better. A 3.5 caliber long slug (1.5"+ long for a 45 caliber) will need a 18" twist, a 20 will work as will a 16. But the twist for best accuracy will likely be in the 16-18" range. Bullet LENGTH is the determining factor. A 550 grain 45 bullet that is a blunt round nose will stabilize in a 20" twist. A more streamlined bullet of this weight will require a 16-18" twist.
I believe the importance of having a *uniform* twist or a twist with a positive gain at the muzzle did not become critical until the advent of the bullet. Makers found that a positive gain shot better than barrels made with their likely questionable "uniform" twist guide they had used for the round ball or had made for use with the longer bullets coming into vogue for target use by the 1830s-40s.
Long winded but its my take in gain twists and why the became popular in the 19th century.
Dan
