If you've played any Multiplayer online, chances are you've run into a player that's a higher level which may have deployable turret. You can upgrade it too, and the top part shoots rockets.
Modeled by Erich Beckmann , Check out his polycount [link]
My Blog: [link]
Future version of the Browning Light Machine gun xD
The browning was originally an anti air gun... maybe you can apply this gun the same in your game.