I would change that answer to very, very, few. Not many barrel makers produce them and not many competitive BR or "F" class shooters care to experiment too much. I have one 308 barrel on a HBR which gains from 16 twist to 13.5 and it is one of the best I have had on a hunter rifle. I have one 6.5 which is also as good as any barrel I have had. The thing is, while a gain twist may work as well, it isn't going to work any better. Also, while there is no evidence that a gain twist will reduce pressures, there is no evidence that it will not. Velocities do seem to be a little better but again, I don't have enough data to say for sure. I prefer a moderate gain; say 20% or so. I will say, the only reason I have used the gain twists is because a friend of mine makes them. He cuts a very good barrel, whether uniform or gain twist.
Some makers have experimented with a very slight gain (start at 12.5 and finish at 12 sort of thing) but I've never heard anything positive or negative about the concept. WH
