I have no idea why Ruger does it.
In fact it was claimed one could eliminate the rifling over most of the length of the barrel without much effect.
Many black powder rifles have pitted barrels and eroded rifling towards chamber, but shoot great.
My own experience with a few "fast twist" barrels has lead me to believe that such might be beneficial.
Progressive twist, gradient twist, increasing twist, gain twist -- whatever one wants to call it -- would likely be easier on bullets in certain applications.
The most prominent offenders in my testing were 1:20" twist .444 Marlin barrels. At one time, I had about 35 recovered bullets from those barrels that very clearly showed that they were 'skidding' across the rifling before finally biting and spinning properly. (Copper or brass jacketed, as well as cast bullets -- all sized for the barrels.) The heavier the bullet, the more pronounced the markings.
Yet, recovered bullets from the exact same loads showed no sign of the 'skidding' in two 1:38" twist barrels.
I've seen similar, albeit less pronounced markings on bullets from several other barrels, including: 1:7" twist .22 caliber, and 1:9" twist 6mm.
At least for .444 Marlin, that was the icing on the cake to convince me that 1:20" is too fast of a twist for the cartridge. The math (stability calculations) had already shown me that it was excessive, since a bullet in excess of 450 gr should be stable at velocities as low as 1,200 fps; but such a bullet could not even be loaded, cycled through the action, or chambered in the rifles with those 1:20" twist barrels.
With 'standard' twist rates, conventional rates seem to be fine. But if one wants to really spin something around, I think gain twist is definitely the better option.
