To help clarify what a railgun does for ya scout dog is simple.
Its two rails, one charged positive and one charged negative, you take a balisitic projectile and put it between the rails and it goes like a bat out of hell depending on the amount of energy is putting into the rails.
Right now the US navy is developing the technology to have very similar to what the movie displayed in terms of a railgun. I cant remember what amount of power there looking at ut i know its enough to send a projectile at mach 14-18 with a huge range, like 500 miles i think in a matter of minutes. The thing they dont show or tell you in the movie is that the destroyer probably used its only shot for the day since that design of destroyer isnt fully meant to handle the power requirements of the railgun, so it have to constantly run its engines to fill the capacitors. The DDX project was intended to be the design platform in the future for the railgun tech, but the DDX has been scrapped after i think 2 ships have been built. Right now there is talk of building a battleship size class of ship, not for armor or anything like that, but to hold alot of capacitors and nuclear turbines.
Right now there is one major flaw in the railgun, its the fact that the rails after the projectile fires through electromagnetic forces, bend outwards due to the friction caused by the rapid acceleration. Also the reason why its not on aircraft is not due to recoil since there is little recoil in a railgun (no explosive forces), its because a fighter can't fit enough capacitors on it to power one.
The shell itself is not explosive it is just a metal shell, right now i beleive they are going for tungsen rods, the damage comes from the kinetic energy. Remember it is:
Force = mass x velocity^2
