Fostech Outdoors had a prototype bullpup bump firing stock for the AK platform on display at Media Day. Its design was striking, to say the least, and most people will either love or hate it. Personally I loved the look.
When the trigger is pulled the AK moves backwards with recoil. The operator keeps constant forward pressure on the foregrip, push the gun forward. If you keep applying pressure on your trigger finger, the trigger will be depressed again as the gun moves forward to its original position. Repeat until the magazine is empty.
In order to accommodate the AK magazine, the pistol grip is mounted at a slight angle. This configuration is reminiscent of the bullpup Bushmaster Arm Pistol.
I hope Fostech bring this product to the market. It would make for a great plinker.
UPDATE: I incorrectly stated that this gun uses a return spring. This is obviously incorrect since as many people wrote below in the comment, such a system is not BATFE-legal. I must have remember incorrectly.