I have a 243Ai with a 1-8 twist Hart 26" and absolutly love it , great long range varmint and target caliber with the 105gr A-max or 107gr Sierra Match King its a sweet long range rig out to 1000yds and with a 95gr Nosler Partition its an awsome deer round.
My rig seems to like powders in the 4350 range , its leaning to the VV-N560 real hard. I have run some 75gr Hornady V-max's out of it at max speed ( 3600+fps) with supper accuracy and the pop it puts down range is way high on the splat factor
This rig seems to hold a true 1/2 MOA out to around 600yds which is the max I've shot it so far , but my best 400yd groups is 1.5" for 5 shots. and that was with 105gr A-max bullets. It shoots the Sierra's well but not as well as the A-max's.
As for the powder , My accuracy load for the 105's is 47.5grs of N-560 which leaves the muzzel at just under 3400fps. the best i get with the 4350 is about 3300fps with a load of around 47-48grs , I'd have to dig out my load books to make sure.
one of the load cards I have here shows a super group with N-550 and a 75gr V-max bullet velocity is at the 3700fps mark.
I did alot of load testing for this rig and used index cards taped to the target so i could later transfer to a load book just haven't yet.