Thanks for reminding me of that 'gun'. I was recently noodlingly around with the question "Can the Navy's rail gun launch projectiles into orbit?" [1] The production gun is expected to us 64 Megajoules. The challenge is that you have to impart all the energy you can before the projectile leaves the gun (because you then lose your source of acceleration) and drag (or air resistance) is proportional to the cube of the velocity. So the faster you try to make something go, the harder the air resists it moving.
Using the math from "Rocket Propulsion Elements" [2] it seems like for anything other than a needle shaped projectile it would not be practical.
Using the math from "Rocket Propulsion Elements" [2] it seems like for anything other than a needle shaped projectile it would not be practical.
[1] http://www.navytimes.com/news/2008/01/navy_railgun_test_0801...
[2] http://www.amazon.com/Rocket-Propulsion-Elements-George-Sutt... Pretty much everything there is to know about rockets that isn't classified.