[danarmak]

A railgun still obeys Newton's third law. The gun would need a mass much larger than the bullet, or it's the gun that will fly away from the earth (so it won't be reusable for future bullets). This makes putting the gun into orbit too costly, or else the bullets too small.

It's probably most cost efficient to de-orbit the entire satellite, and include a rocket that fires upwards when it reaches a certain low height, to make it exceed its natural terminal velocity.

[BurningFrog]

Yeah, the real "killer app" is throwing asteroids that are already in space on your enemies.

[danarmak]

Most naturally dangerous asteroids (near passes) are being publicly tracked, so people would notice you attaching a rocket and nudging it off course. Bringing a whole new asteroid from the belt is feasible (or will be soon), but it takes a lot of time, so you need to do it in advance. But if you put an asteroid in orbit, astronomers (at least) are going to notice and ask questions, which they wouldn't do for a satellite with a cover story.

[BurningFrog]

OK, the world will know, but if no one can stop it, that isn't worth much.

I expect an arms race in this area.

There are also plenty of rocks on the Moon, but I don't have a good feel for how practical weaponizing them would be.