The heavily modified AR-10 used in the Metal Slug series.
Supposedly it fires 7.62x51mm. The launcher is supposed to fire 40mm grenades.
These factors are completely ignored, however, as it fires bullets the size of a 25x137 M793, and the launcher is capable of firing just about anything you find, regardless of wether it fits inside or not, be it rockets, 4-gauge shotgun shells, "grenades" that are pretty much tank shells, bouncy balls, lasers, bombs with wheels, bombs with legs, heatseeker missiles, flame bursts, robots, lightning bolts and pretty much every can of whoopass you can think of.
They also somehow fit a whopping 200 bullets in those lil' magazine they have.
No wonder they destroyed tanks, planes, blast doors, battleships, nuclear submarines, robots, aliens, motherships and extra-extra large iterations of everything I just mentioned with just one or two guys using these things.
This was partly done in PMG, and partly photoshopped.