I have an 8 1/2 twist barrel, and it shoots a 15 grain, but no heavier. A 7 twist shoots the 18 grain I have made at least. I think you can get down to a 5 twist. Interesting to note that I have exceeded the bullet weight with powder weight. The 15 grain must be pushed to at least 4300 with the 8 1/2 twist to shoot well.
I think it will take tungsten powder core bullets to go beyond 200 yards, hopefully matched with the 7s boattails I plan to make when I get the dies.
If anyone wants to do some math, my 17.5 grainers are .144 in diameter, the 6s ogive is .235 long, and the overall bullet length is .525. The meplat is .050.
russ