Skip AI,
I would think you would come close to matching the velocity of a 223 with same weight bullets. The larger bore and shorter baring surface on the barrel will really free up the velocity. Would be a very long barrel life round.
If the rifle was built properly with a properly designed chamber, I would say the 6mm PPC would give you any more accuracy or consistancy. With a little sorting at the loading bench and case trimming you can get extremely tight groups using the Fireball cases.
I have had three XP-100 factory handguns over the years and the worst shooting one of the bunch was a 3/4 moa handgun at 100 yards with a 4x scope.
The best of the three I still own and always will. It is a legit 1/4 moa handgun out to 200 yards using a Burris 2-7x handgun scope.
I am just getting ready to build myself a light weight walking rifle in 221 FB and I will be very unhappy if it does not hold 1/4 moa out to well past 200 yards with quality loads.
I would not worry to much about the FB case. Get the reamer designed properly with a throat designed for accuracy and a snug neck and body and you will be very happy with the results I assure you.
Kirby Allen(50)