Twist is one factor but so is jacket construction. J4 jackets tend to be thinner than Sierra jackets. The solid copper bullets should take more RPMs than the Sierra's. When I first started shooting prairie dogs used the Speer 22 Cal. 52 grain flat base hollow points in both the .223 and the 220 Swift. They were only available in 1,000 bulk packs. It didn't matter how fast you pushed them they performed superbly. I went thru 8K without issue than Speer introduced the TNT bullets and they changed the jacket to the TNT design. After that I could blow them up with a 9 twist .223.