It's called the Metal Storm. This Australian-made, U.S.-funded behemoth of a cannon uses the same idea behind a Roman candle to fire round after round out of its 36 barrels. The prototype managed to achieve a maximum rate of fire of 1.62 million rounds per minute as it fired 180 rounds in a 0.01 second burst. At its peak, it can send, almost literally, a wall of 24,000 9mm rounds moving at Mach 5 that can eat through any armor it faces.
In 2007, the U.S. Navy announced that it would buy the Metal Storm grenade variant, but shy of that... nothing. The first prototype was created in June, 1997. It's been over 20 years now and it's never been fielded in combat.
In short, the reason why this potential game-changer has never seen combat is mostly tied to legal issues surrounding contracts. But there's also the rarely-brought-up question of, "how would we use it?"
Originally developed by J. Mike O'Dwyer under a company of the same name, Metal Storm Limited, the technology behind how the gun electronically fires caseless rounds has been tossed between several countries' governments and many more companies, acquiring the intellectual property and trademark claims along the way. The rights ultimately landed in the hands of Australian-owned DefendTex.
Owning this patent not only keeps the original Metal Storm under their corporate thumb, but also any variations, including the 3GL grenade launcher, which fires three rounds from one of its four barrels in seconds, and the MAUL (Multi-shot Accessory Under-barrel Launcher), an under-barrel 5-round shotgun using the same technology.
These could revolutionize drone warfare.
(Metal Storm Limited)
Outside of legal issues, there are some very obvious downsides: cost and weight. Its applications, as is, are very circumstantial. It's extremely heavy and requires plenty of prep time to set up effectively just for a single use. Then, there's the insane amount of money that goes into fully loading it, only to have it waste nearly all of its ammunition.
Aiming this thing is also a challenge. It was originally conceived to remain stationary and to be used in setting up ambushes. Anything in its line of fire would be effectively turned into a paste, but by stepping a few feet to either side, the target remains fully composed solid.
On the bright side, if you were turned to paste by this thing, you'd be obliterated in milliseconds and wouldn't even have a chance to blink.
(Screengrab via YouTube)
These extreme limitations aren't factors for the easier-to-sell versions, the MAUL and the 3GL, which can all easily be manned, moved, and loaded. The MAUL can easily be modified to fire less-lethal rounds and has been issued to Papau New Guinean prison guards while the 3GL has been fitted onto the Cerberus UAV with 3 rounds in a single barrel.
There is still hope for the Metal Storm's technology. The caseless, electronically fired, multi-stacked rounds will change future wars. But, for now, don't hold your breath on getting your hands on one of the 9mm versions.