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.
Including cooling because casings actually absorb quite a lot of the heat generated by the propellant iirc (which is then easily removed from the weapon by ejecting the spent casing)
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.
For the last few rounds, there are IIRC three tethered dummy cartridges that are pushed by the spring into the slide, so that the last few rounds are properly fed and can't rattle around.
Cartridges aren't gravity fed through the slide or anything. They are pushed through and into the receiver by the pressure of the spring, same as any other gun.
First solution might look over engineered (in not qualified to decide as I'm german) but probably works in a lot of conditions (running, jumping, sudden movements, shaking, like a gun may experience during use in conflicts) whereas the slide maybe only works when the sun is shining, the birds are singing and the people are picnicking or such.
The slide works in all conditions afaik. It's not gravity fed or something. It's full of cartridges under presure from the spring, same as any other gun, and rounds are pushed into the receiver one by one. IIRC three dummy rounds are tethered to the end of the spring, which allows the spring to push the last few cartridges into the slide and out of the magazine.
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.
There were far simpler caseless mechanisms devised. But the G11 fired caseless ammo, rotated it 90 degrees and more importantly had a burst mode where the barrel and magazine recoiled together continually throughout the 3 round burst, only returning to battery after the 3rd round was fired. That latter one added a lot of complexity.
It's also pretty fast at about 2000 rpm in burst mode. If they hadn't used caseless ammo this would have been a really reliable gun.
Loading it by turning the fold out knob is a bit wonky though. No satisfying chambering clanks like with the M14 or G3. Probably a plus in real combat.
It's not even that caseless. The bullets sit in a rectangular box that contains the primer, propellant and a plastic cap to center it. The ammo really is the worst part of that thing.