I have 7, 8, and 9 twist .223's. I find 8 twist to be a pretty good compromise. I usually shoot 55 - 65 grain bullets in all. I don't quite get the accuracy I'd like with 69 grain SMK's in my 9 twist gun, but I'm not ready to say it's the twist, I haven't really worked with those bullets in the rifle so much. I've found no problem at all shooting 55's in my 7 and 8 twist rifles. I've never shot anything lighter in .223.
