US Patent 10488136 Selective fire firearm systems and methods

A selective fire firearm may be provided having a fully automatic open bolt and a semi-automatic closed bolt mode of operation. The firearm may include a trigger group having a trigger prop that ensures that, when the trigger is released in the fully automatic open bolt mode, the bolt carrier stops in an open bolt position held by an open bolt sear. The trigger prop may include a foot that, following a first trigger pull in the fully automatic open bolt mode, slides under and props a rear end of the trigger in a position that allows the open bolt sear to catch the bolt carrier when the trigger is released and propped but prevents the trigger sear from engaging the hammer.