US Patent 10001336 Auto sear actuation linkage for compact AR-platform firearm

Disclosed is a mechanism for use with firearms having compact bolt carrier designs without modification of or connection to either the upper or lower receiver. It provides an auto sear trip linkage that is independent of both the upper and lower receivers and which may be used as a “drop in” unit to allow an upper receiver assembly otherwise configured for semi-automatic fire to be used in full-auto mode with a standard, full-auto capable AR-pattern lower receiver.