MarcoPolo,
Quick question for you here first; what's the chamber? We know it's a 223, but what reamer? A standard 223 is going to be a problem for you, in that you're not going to get the full use out of the 80-82 grain bullet that the rifling will stabilize. I'd have suggested (and hopefully, this is what you have) a Wylde chamber, which is what the vast majority of competitive Service Rifle shooters are using. The difference is primarily in the throat length and leade angle, but this will allow these longer bullets to be seated out to around 2.440"-2.450", and give you a bit more case capacity. As far as the twist goes, you've got it nailed, and the 7.7" will work just fine for anything short of the 90s. Varget, N540 or RL15, and you're set to go to the range.