It's the HK G11 and FN P90. Both guns have a magazine where the cartridges aren't in line with the chamber so each cartridge needs to be rotated before it can chambered. HK decided to commune with ancient clockwork gods to learn the secrets of kraut space magic while FN just went with a curved ramp.
As far as I know, the only thing that keeps the cartridges moving down the slide in the P90 is the magazine's spring pressure. Are they more likely to jam compared to conventional receivers?
edit: but then, with so few moving parts, a confident smack might unclog the slide. I don't know, I'm European.
FN P90's magazine turns the bullets 90 degrees by turning them with its feed geometry. It kinda looks like a slide for bullets if you squint hard enough.
HK G11 uses about as many parts as in an average Swiss watch to do the same thing.