Now I will add some more to this. First off I feel this applies more to a lead bullet shooter than a jacketed bullet shooter but some of the why’s and why not’s do overlap. With a gain twist barrel the bullet cannot go to sleep. The rifling is always putting a fresh bite on the bullet as it goes down the bore of the barrel. This is why I always go back to a cut barrel being better than a button barrel. A cut barrel even with a straight twist is more uniform and consistent than a button barrel. With button rifling the button can hit a hard spot/soft spot in the steel and it will slow the button down. The button could speed back up and do the twist it’s suppose to be doing but either way you end up with a non uniform twist and it the twist keeps getting slower towards the muzzle. These two things are a accuracy killers and lead to consistency problems/fliers etc… I feel even a slight gain twist will help accuracy wise and not hurt a jacketed bullet shooter as well. For the most part I would say there is no velocity gain in a gain twist barrel with the same load. What has been conveyed to us and it goes back to Popes 1st point is that shooters have noticed that they can run a slightly heavier powder charge vs. a shooter with a straight twist barrel. As the bullet is starting easier into the rifling my only guess is the pressure isn’t spiking as fast or is delaying the pressure curve. Hence forth they can get more velocity out of the gain twist barrel. I feel pressure is pressure and that the twist doesn’t have anything to do with pressure for the most part but my only guess is that the gain twist like I said earlier is delaying the pressure curve. So you don’t see problems as early like hard bolt lift etc… Also it’s noted that even now a days our military in some 20mm and the 30mm barrels like on the A10 Warthog ground attack aircraft have gain twist type rifling in the barrels.
Also cut rifled barrels have a more uniform consistent twist than a button rifled barrel. This is because sometimes as the button is being pushed and or pulled thru the bore it can slip and the end result is the uniformity of the twist is not consistent through our the length of the barrel.