If you are going for absolute accuracy with 55 gr projectiles (and there really are some good ones out there) then 1x9 non-chrome lined either 4150 or stainless (nitride or not) will be your best bet. Chrome lining is for barrel longevity under adverse conditions. Nitride is also for the same adverse conditions, but does not have the negative impact on accuracy because of uniformity of the treatment.
The 1x7 twist was a compromise by the military to try to stabilize a specific projectile. From what I understand the military was all set to go to 1x9 from 1 x12, when one projectile in the inventory (I think it was some sort of tracer/incendiary actually) was found to fail to stabilize. So they made the twist faster, which actually made the groups open up with a number of loads, but brought all into the specification they were looking for.