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ZM and OA are different systems all together.
I wish I could find my pictures quick.
ZM has a short gas tube and a long key that goes over it, the long key also has the spring that goes over it.
OA has a standard style gas tube and the recoil spring is above that.
I've handled a ZM in the gun store, it seems kind of stiff charging it, it still uses a charging handle in the standard position, it is different though if my memory serves me correctly.
There is a guy on ar15.com who made an AR pistol with a similar spring config as ZM, it turned out pretty slick, he also added a forward charging handle which I believe is non reciprocating. see attached.
Jon
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