Owen cleverly fashioned the drum magazine from an automobile crankshaft’s harmonic balancer. He drilled holes into the balancer to hold each individual cartridge, essentially creating a revolving cylinder with individual chambers. The magazine/cylinder held 44 .22 short cartridges.

The prototype lacked a traditional trigger. Instead, Owen added simple a thumb trigger that he made out of spring steel. When cocked, the trigger held the bolt back. Depressing the trigger released the bolt, allowing the weapon to slam-fire.

https://medium.com/war-is-boring/the-very-first-owen-gun-was-fashioned-from-car-parts-b69d231b92b6