Tango27:
Chuck's answer is right on the nose! Very good info.
You will find that quite a few target/varmint barrels come with a 1:8 twist... largely because this is a very good, overall twist rate for the longer (heavier) bullets. I think that Kreiger pushes a 1:7.7 twist barrel?
To be honest, the 1:7 twist rate is faster than needed for almost any ammo you will use... Granted, as Chuck pointed out, the OAL of the bullet is the determining factor and consistant density plays in as well, but given you are limited by the dimensions of the magazine, I would say that you will be more than pleased with the 1:8 twist rate.
BTW: The Greenhill formula (with a constant of 150) is intended for velocities much lower than what the AR/M16 shoots -- remember that the stabilization is a result of the actually speed of the bullets spin (RPM) and that the velocity is a determining factor in that speed... Most would suggest you use a constant of 180 with the Greenhill formula for your purpose.