Franklin Armory BFSIII™AR-C1
Otherwise known as the legal fun switch.
It allows you to shoot twice with one full function of the trigger. Fires once on the depress, once on the release.
Listing is for a binary trigger for an AR 15 lower.
All parts in picture included.
If needed. I can install the trigger. Just bring the lower, preferably with no upper and I will install it.