@ninjadork You are applying the physics of gunpowder (explosive-based) weapons. I think this gun is more like a railgun, an electromagnetically accelerated high density metal piece. Much higher starting velocity. Maybe that's why only slave-race can use it, they are much tougher than humans, as was explained before.
Yes, you are right about the ammo falling in a parabolic way and all, but this is fiction. You also left out the planet's curvature, which even on the largest known habitable planet would cause problems at this distance.
Also, that ammo had some electronics embedded to be able to enter the force field, so it's totally possible it had additional propulsion along the way. Or whatever, just use your imagination, man, dont say "it's not possible because current developements dont allow it". If you are like this with everything, you will never be able to enjoy these works.