The larger powder volume in the BR will give higher velocities for equal barrel lengths and operating pressures.
Go here to get an estimate of velocity changes with barrel length: http://shootersnotes.com/calculator/velocity-estimator/
If you are starting with the .473" bolt face, the BR will be easier since you won't have to swap bolt heads.
If you are starting with a bolt for the 7.62x39 (~0.440" bolt face) then the PPC is easier.
If you are starting with a bolt face for the .223 then both are equally challenging, leaving the decision to other factors.
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