I think a lot of people generally don't realize that impact energy is far more dependant on velocity than projectile mass (generally related to the shell diameter).
So, just an aside:
E=(1/2)*mass*velocity^2
Doubling muzzle velocity will increase energy by a factor of four, while doubling mass will only increase energy by a factor of two.
This is why, for example, 5.56mm assault rifles are far more lethal than a 9mm pistol. The pistol fires a beefier round but the rifle fires its round with a much higher velocity.
The aerodynamic shape of the round determines how much speed the round will lose with distance.
The type of round will also affect what sort of muzzle velocity you can get.
A 75mm Sabot AP round will achieve a higher muzzle velocity than a normal 75mm AP round. While the old-style AP shell is fitted to the barrel diameter, the sabot shell is actually smaller than the 75mm diameter of the barrel, but it is held in place by a "discarding sabot", a sort of boot made of lighter materials, which fits the barrel and falls off after the round is fired. Thus the same explosive charged can be used as for the standard AP round, but since the sabot round has less mass, it will be accelerated to a higher velocity.
http://www.arl.hpc.mil/SciVis/images/avco131.gifAlso, rounds fired from rifled barrels which stabalize the shell by causing it to spin about its length will have a lower muzzle velocity than fin-stabalized rounds fired from smoothbore barrels, because some of the energy from the explosive cartridge is used up in the rotational acceleration of the round, whereas in the latter case, all of the energy accelerates the round straight forward.
