The USRG Portable AA-Gun. Just something I put together on LDD in about twenty minutes. Those wheels on the sides are the ammo belts, and can easily switched out with fresh ones. It is remotely controlled, or can be programmed to fire on any aircraft with enemy IFF transponders (Identify Friend/Foe).
You should put a link to the page of my .50 cal, seeing as how you got the speargun idea from it. Onto the gun itself, I like the portability of it. It looks like you could just pick it up with one hand and ride away on your skateboard.