Abstract: An auto sear for a firearm having a lower receiver with a trigger, disconnector and hammer, and an upper receiver with a reciprocating bolt carrier group. The sear includes a body pivotally mounted within the lower receiver rearwardly of the trigger, disconnector and hammer. The body includes a forward-most planar shaped portion in which are configured each of lower and upper trip engagement location. Upon the hammer initially striking the firing pin, the bolt carrier subsequently displaces rearwardly following cartridge discharge in order to rotate the hammer downwardly into contact with the lower trip engaging location. Upon a return forward displacement of the bolt carrier, an underside ledge thereof contacts the upper trip engaging location in order to pivot the lower engaging location out of contact with the hammer, permitting the same to rotate back into striking contact with the firing pin.