The One-fifty-one is a ROW or Remote Operated Weapon, currently in use with the U.S. Military, it's mounted on the top of Strykers to provide fire support for ground troops, my 151 is Minifig scale and my tank is Mini scale so it's rather large for it.
About this creation
IN this blurry shot you can see most of it, the main picture is better for seeing it, but you can see the single stud piece in the middle, on the real 151 there are optics there, two in fact but obviously I couldn't put a smaller one on, you can also see the Ammo box on there on it's left side, and smoke grenade launchers as well, there are supposed to be 3 per each of them but again, pieces are too big.
you can see how large it is in comparison to the tank.