I only saw them in .223 but I assume you could use it with any caliber you could get/make a barrel for. The barrel and most other parts are standard.
I dont know why you never saw these, they have been selling (unlike ZM) since the early 90's and tons were sold. As stated they are the OA series rifles/pistols (pistol pictured) made by Olympic Arms in Olympia, WA.
Right from the Quarterbore Forums,
http://www.quarterbore.com/ar15m16/oa93.htm