Thinking about railguns last night. It suddenly became clear that I had made a mistake. I computed muzzle energy of the conventional cannon as M * V . That's not right. M*V is momentum. Kinetic energy is 1/2 MV**2. So doing it right, the conventional cannon has a muzzle energy of 7,801,736 foot pounds. Which compares more favorable with the railgun at 23,603,200 foot pounds.
Onward to phasers.