Brass would seem to have a low density hence result in low sectional density... Obviously we want to be going in the OTHER direction... aka higher sectional density.

I was just thinking:
You know how they make those chocolate covered peanuts.... I'd like to get me a copper coated lead ball....haha

Or another way to visualize it is: a perfectly round hollow copper ball, drill a very small hole in the wall and inject it with molten lead. The port or orifice to feed the lead in would create the smallest imbalance... but, I would think it would not affect accuracy up to 50 yards.... hmmmm.