I remember seeing it in the movie Demolition Man, being wielded by Wesley Snipes. The real thing is just as fantastic as the movie portrayed.
Remember Metal Storm? The G11 was there first. H&K developed caseless ammo for the G11. Their primary focus was to get a three round burst to be accurate. They surmised that the problem of accurate burst is due to the recoil of the weapon. So they developed this system to fire a three round burst all in a single cycle of recoil.
Similar to the FNH P90, the H&K G11 was the predecessor. The magazine had ammo oriented 90 degrees vertical rather than horizontal like in the P90. The round would be chambered and rotated 90 degrees to be fired.
It was chambered in 4.73mm x 33
The Germans even went as far as to design a pistol for their caseless 4.73mm x 33
Sadly the project was shelved and never put into mass production.