Open bolt
FIREARM MECHANISMS WHERE THE BOLT IS IN THE REARWARD POSITION (OPEN) WHEN READY TO FIRE
Open-bolt
A firearm is said to fire from an open bolt if, when ready to fire, the bolt and working parts are held to the rear of the receiver, with no round in the chamber. When the trigger is actuated, the bolt travels forward, feeds a cartridge from the magazine or belt into the chamber, and fires that cartridge in the same movement.