Just off the top of my head, I'm no engineer or anything (can't even remember the last time I read cr4.globalspec.com
) but, commonly accepted practice in ballistics is to ignore potential engergy, thermal energy and concentrate on kinetic energy. KE=1/2 m v^2, which both bullets have the same amount when leaving the muzzle of the gun. The effiency of the transfer of this energy is the key to the solution. If we assume that the lead bullet penetrates and does not exit the wood, then it has transferred all of its KE to the wood. If the rubber bullet does not penetrate the wood, then it has only transferred a portion of its KE to the wood. The diameter of the bullets only dictates how large of an area their respective amounts of energy are transferred. On the other hand, if the bullets are fired at sufficient velocity that both bullets penetrate the wood, it is likely that the lead bullet will pass through entirely and the rubber bullet will remain lodged in the wood. In that case the rubber bullet transferred the most energy to the wood block, since it was stopped by the wood, it expended all of its KE in the wood, while the lead retained some of it's KE on exiting the wood. If the bullets were fired at a velocity that resulted in exiting the wood in both cases, then internal ballistics would be a factor and bullet diameter would have to be considered.