that 1 in 12 twist wont do very good with 50 gr bullets and heavier, better stick with 50 gr and lighter with that loose of a twist 40 and 45 grainers are the most accurate. I have a m700 adl is .223 with a 1 in 12 twist with 55 grain bullets at 100 yards the best groups I could get was 1 inch but at 200 yards it openned up to 5 inches, that 1 in 12 just doesnt stablize that grain of bullet enough for that distance, I ended up going with a 45 grain bullet and my groups at 200 yards shrunk down to just under 2 inches with that rifle i stay with the lighter rounds, I recently bought a savage 10FP in .223 with a 1 in 9 twist it does great with the 55 grain bullet holds under an inch at 200 yards.
Heres a pic of what my 10FP does at 200 yards