Jeez. Stackfire guns. Alright.

Imagine a lot of barrels. Its more than 1 barrel. Like 20-30 barrels.

And here's the distinctive part. They are not magazine fed, at all. The bullets are already inside the barrels when you reload. Hundreds of them. Dozens in each barrel.Just stacked in there, specially, with caseless ammunition. The bullets are sort of integrated into the barrel, its all very strange.
Anyhow, these bullets aren't ignited in the typical hammer and primer way that most bullets are. They are ignited electronically. And as you can imagine, when you combine literally lightning fast ingnition, multiple barrels, and cut out any rechambering time, the bullets come out at a very fast rate.
Its kind of like a bunch of roman candles. Except faster, and deadlier.

Nearly 10 years ago today even, there were prototypes that could fire 1,000,000 4mm case less rounds per minute.
There are even ones that can fire 30,000 40mm grenades per minute. Its quite insane.

Here's a few things so you can sort of get a decent idea of these weapons.
www.youtube.com/watch?v=d8hlj4EbdsE The real Metal Storm prototype. Probably the best video that shows these weapons in action.
www.youtube.com/watch?v=iflmHVj1dzU. This is the "Typhoon" seen in Crysis 3. While it is a videogame, they did a very good job of making a realistic representation of one. Its worth showing.