rifling can be helpful in allowing bullets to begin moving forward earlier.
A gain twist
barrel engraves and spins the bullet up gradually.
The benefit of gain twist
in an extreme high-pressure/high-velocity modern revolver like the M460XVR is similar in that it allows the bullet to transition into the rifling more slowly from a nonrotating state instead of being forced to "spin up" instantly when it crosses the barrel/cylinder gap and slams into the rear of the barrel at the very instant deformation potential is at its height.
I strongly recommend combining a gain twists
with some of the products discussed here to build better rifles.
In fact, the Carcano was the first 6.5mm military rifle ever adopted and featured a unique, gain twist
Consider gain twist
rifling for applications that warrant it as these are more readily available now.
By this time Bartlein was also offering gain twist
barrels beginning with a slow twist and incrementally increasing toward the muzzle should last longer than barrels with uniform fast twists.
I talked about this most recently in my December 2016 article, "Gain Twist
Barrels For Palma." Those of us who write frequently for American Gunsmith sometimes find our articles "in the pipeline" for awhile from the time they're written until they are published.
barrels will work equally well when mounted on bolt guns or gas guns.
About a year later, I was building a Palma AR-15 for a customer with a 32" Bartlein gain twist
5R barrel and a tight bolt rifle chamber.
Standard centerfire grooves are 0.2240", though I use a custom groove of 0.2245" on my 90 grain gain twist