This gun doesn’t work on land? In Space?
It’ll work on land. Space is trickier, I would guess because of transfer of momentum.
I'll work on land just fine (and space, too, I'm sure). I can imagine, though, that if they want to test it. it's probably better to do it where an accident isn't likely to shoot the projectile into someone's house, or where they don't need to spend half a billion dollars just to get the thing into orbit.