Ideally, you want the bore size that causes the balls to lightly touch the barrel in two spots on opposite sides of the ball, usually at the seam. This will give you maximum consistency. Ultimately, consistency depends on having paintballs that are consistent. A larger bore is more forgiving than a smaller bore when it comes to paint that is not perfectly round. A larger bore also prevents barrel breaks due to weak seamed paintballs. The guys that are underboring for their paintball size, are depending on having good quality, consistent paintballs.
Except for the Automag in front, its usually the man behind the equipment that counts.