Runner,
Just a couple of thoughts.....be a little cautious about using the wood out of limbs to make anything.The cellular structure on the top part of the limb grew under tension,and that on the bottom grew under compression.This makes it dimensionally unstable and prone to twisting cracks when worked into any wooden object. You might be better off selecting a standing tree about 8-10 inches in diameter and using a chunk cut out of the butt maybe a foot or so above ground line.
Hickory is great.I've seen a zillion mallets made out of hickory. IMO hard maple might be a little better simply because the cellular structure is what we call diffuse pourous as opposed to ring pourous as hickory and oaks are.Wood,when under high stress will tend to shear along the grain,and ring pourous species will shear a little quicker than diffuse pourous species.Doubt,though,that you would ever put a mallet to the point that it would shear along the grain.
Bet that Osage Orange would make a great mallet! We don't have it down here in the Deep South,but I've heard a lot of good things about it.Thunder and lightning won't bust it,I'm told :m2c:.