US Patent 10107580 Trigger group for semi-automatic firearms

Trigger groups for semi-automatic firearms have a frame, a hammer connected to the frame and movable between a cocked position and a striking position, the hammer being biased toward the striking position, a trigger element connected to the frame and movable by a user between a forward position and a rearward position, a selector connected to the frame and movable between at least a first position and a second position, a plurality of retention facilities each operable to selectively restrain the hammer in the cocked position, and when the selector is in the first position to enable discharge of the firearm in response to movement of the trigger to the rearward position, and when the selector is in the second position to enable discharge of the firearm in response to movement of the trigger to the forward position after movement to the rearward position.