The destructive power of this gun comes from the energy of its momentum (mass x velocity) -- all the energy expended to accelerate it needs to go somewhere when it reaches its target. Same principle as a handgun. The difference is in how the projectile is accelerated.
It has a very illustrative question: Would you rather be hit by a 1000 kg vehicle going 1 meters per second, or by a 1 kg meatball going 1000 meters per second?
I'm not confusing anything, but I wasn't trying to be scientifially rigorous, either. By "energy of the momentum" I was referring, admittedly imprecisely, to the fact that the kinetic energy is a function of the momentum. Or, to be precise, E_k=p^2/2m. At least as I understand it.
Huh? Energy and momentum are two _different_ quantities related to the movement of an object. I was talking about using the energy _instead_ of the momentum as an indicator of damage.