Space

You'd have to make it in a hyper tube or it'll just vaporize, which is damn near impossible to keep depressurized even without the rail gun. Or you could put it in a giant sabot, but those are expensive and their own technical challenge. Not to mention, good luck adjusting it for a different orbit... The power requirements are going to be insane too, which means a ton of huge capacitors

This way, they depressurize a nice pancake shape, spin it up over the course of hours, and let the projectile pierce out through a brake away cap. If they can make it work, it's a better design in every way