Would a naval gun be effective in the va of space?

Would an 8 inch naval gun (like on a cruiser) with a solid projectile be an effective weapon against spacecraft if mounted on a spaceship? Would it even fire? I know there's no air in space but arent the shell casings airtight anyways? And if it did fire wouldnt the shell be deadly for millions of miles due to the lack of gravity and air resistance? How do you think it would stand up to an electro-magnetic gun?