Trigger group for semi-automatic firearms

1. A trigger assembly for a firearm comprising:

a frame;

a hammer connected to the frame and movable between a cocked position and a striking position;

a trigger element connected to the frame and movable by a user between a first position and a second position;

a selector connected to the frame and movable between at least a first position and a second position; and

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 first 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 second position after movement to the first position.

2. The trigger assembly of claim 1 including a first retention facility and second retention facility operable when the selector is in the first position to release the hammer when the trigger is pulled to the second position to discharge the firearm and to retain the hammer in the cocked position while the trigger remains in the second position after discharge of the firearm and after the trigger is returned to the first position.

3. The trigger assembly of claim 2 including a third retention facility and fourth retention facility operable when the selector is in the second position to restrain the hammer in the cocked position when the trigger is moved to the second position, to release the hammer to discharge the firearm when the trigger is moved to the first position after the trigger is pulled to the second position, and to restrain the hammer in the cocked position after discharge of the firearm.

4. The trigger assembly of claim 3 wherein the frame includes a cam surface, and at least one of the first, second, third, and fourth retention facilities has a cam follower adapted to follow the cam surface and generate a movement of the at least one of the first, second, third, and fourth retention facilities to cease restraint of the hammer by the at least one of the first, second, third, and fourth retention facilities.

5. The trigger assembly of claim 3 wherein when the selector is in the second position, the third retention facility is operable to restrain the hammer in the cocked position, and to release the hammer to restraint by the fourth retention facility in response to moving the trigger to the second position.

6. The trigger assembly of claim 3 wherein at least one of the first and second retention facilities and at least one of the third and fourth retention facilities comprise a set of adjacent retention facilities operable to engage adjacent portions of a common portion of the hammer.

7. The trigger assembly of claim 6 wherein adjacent retention facilities are each planar elements parallel to and adjacent to each other.

8. The trigger assembly of claim 1 wherein the selector includes a block element operable to prevent movement of the selector from the first position to the second position while the hammer is being restrained by any of the retention facilities.

9. The trigger assembly of claim 8 wherein the block element is interoperable in response to depression of the hammer by a bolt carrier to separate the hammer from all retention facilities, the hammer operable to move at least one of the retention facilities away from the block element to enable movement of the selector from the first position to the second position.