They are not piston uppers. I owned a ZM for a few years, it's simply a modified buffer system. Instead of the buffer behind the receiver, it's on top of the barrel surrounding a larger diamter gas tube, the bolt carrier is propreitary mated to the said larger diamter gas tube (it's staked on), when the upper is assembled, the larger dimater gas tube telescopes a shorter and thinner gas tube that's fixed to the upper, which channels the gas from the barrel. The return spring in turn surrounds the larger diamter gas tube (with the attached bolt carrier) Gas goes through the short gas tube, pushes the bolt carrier back, when the energy is spent, the return spring returns it to battery.
Same bolt, gas still gets blown into the bolt itself.
Because of the unique bolt carrier, the auto sear has to relocated.
The OA97 does it pretty much the same way, though with much less refinement and aesthetics, it's friggin' ugly. Its folding stock is from Ace. While the stock itself is light and sturdy, the folding mechanism sucks major ass. ZM/YHM uses a modfied FN folding mechanism. Simple and quick to fold/unfold, in addition to the heavy skeleton stock it also offers a LOP adjustable folding stock, it's probably one of the nicest stock made. Cheekweld is not improved and it does not have any storage capabilities (this was well before the Crane/Vltor), they really should cover it with some synthetic material so in freezing weather the stock won't get stuck on your face