A ball block does not hold JUST the ball. It is customary to load the ball and Patch in the block. Therefore, the block holes are drilled to bore size. a .50 cal. rifle uses a .490" diameter round ball. But it will be patched, so you drill the hole to .500" so that the patch and ball will be held firmly in the block. The patch is lubed before the block is loaded, of course, so that the PRB is driven out of the block and into the barrel.