maybejim said:
A copper bullet would be lighter than lead. Why would it penetrate deeper? I would expect it would not penetrate as deep.
There are several factors which affect penetration.
First, a lighter bullet accelerates faster because it has less inertia, so muzzle velocity tends to be a bit higher with lighter loads. Since the energy of a bullet follows an equation generally in the form of -
bullet mass*velocity*velocity (velocity squared), increases in velocity have a greater effect on bullet energy than increases in mass. Making a slightly lighter bullet go a lot faster makes a more powerful load.
Second, copper bullets have a different shape than lead bullets - in order to achieve a similar weight they have to be longer. This subtly changes the sectional mass of the bullet - shifting it slightly toward being more like a spear with more of the mass behind the point than just around it like an ordinary lead cartridge.
Third, the metal characteristics of copper are very different from lead. Copper is more "ductile" which means that the metal holds together more under duress. Copper, for example, can be drawn into long thin wire while lead (due to its more granular crystaline structure) would fragment. Lead bullets will often fragment, and the bullet mass breaks up into numerous smaller, less massive, projectiles which lose their momentum rapidly. Copper bullets almost always retain 100% of their original weight so all their momentum ends up driving the bullet forward and deeper.
