I found it by mere chance while browsing the web, as it often happens, for something completely unrelated.
The Thor A1 is a PDW concept chambered for the standard 5.56 NATO cartridge and which adopts some interesting solutions to achieve extreme compactness and readyness.
All revolves (literally) around the half-moon C shaped magazine, which can contain no less than 50 cartridges. Closed, the ThorA1 is very compact, more or less like a small SMG.
Stock and forward grip are deployed rotating around the magazine by preloaded springs at the press of a button, but the weapon can be used even if closed.
Accordingh to Eggert, the weapon can be reloaded easily with both closed and open stock, and the weight of the magazine acts as a counterweight for the barrel and accessories for optimal balance (at least with a full magazine).
Barrel and action cooling has been taken into consideration, and the reciprocating slide doubles as a pump to grant a flux of fresh air from front to the rear of the gun, where it's ejected from a fin covered opening.
Personally, I'd have chosen the opposite direction, so to avoid having hot air thrown into the user's face, but I believe this would be an easy issue to solve.
The chassis is made of polymer and it can be ideally had in a variety of colours.
The whole gun is designed to be disassembled without tools or any need to remove small parts that could be lost, and the cleaning and maintenance kit is stored in the rotating stock.
The feed mechanism is based on a helical ramp that rotates the cartridge and a pivoting bolt that makes room for the slide, which grabs a fresh cartridge from the magazine and then runs forward, while the bolt locks into position rotating upwards. The nice animation on the website shows the action quite well.
It's the bolt up and down action, “like Thor's hammer” in the designer's own words, that gives the gun its name.
Being just a concept some details, such as spent case ejection, trigger action and the locking mecanism, are unclear (as it is, the gun seems to rely on a blowback mechanism with firing pin integral to the bolt face, like an old-fashioned SMG, but naturally enough this wouldn't be the case were the gun to be manufactured for real).
The designer even thought about a few accessories: a sound moderator with a picatinny rail for a grenade launcher, bringing the “Personal Defence Weapon” concept to a new level.
Even if a few details need some smoothing out, overall it's a well devised gun, with some very innovative concepts and a very cool look, which never hurts.
You can see the Thor A1 for yourself at http://www.pascaleggert.de/ThorA1.html
where you can also find other concepts, among which that of the Epic A3, a .50 BMG rifle capable of steering ammo via quad electromagnetic coils. A far more science fiction system, but interesting in its own way.