It could basically be a weapon that will go in 1 direction for a while, and lose speed before it goes back the way it came, increasing speed until a max. It could also use an up or down direction to give it vertical movement as well. Thanks to this mechanic, I think it could be able to deal more damage if it hits on it's returning arc. It could also be used as a hortizontal AA, which you can shoot it at degree 90 or 240(or -90), and it'll return back to the turret. If it hits the turret it came from, that turret is reloaded automatically. However, if after it's returning, it does not hit the turret when it's Y = the turret's y, the turret should start reloading. This could be a cool gun kind, and I know that there are many who could do this. It'll be an interesting idea, so I hope somebody considers it!