6. The Binary Firing System

Nearly all bump stocks, trigger cranks, and similar products for accelerating a semiautomatic rifle’s rate of fire have disappeared from online sellers — with one exception.  The Franklin Armory Binary Firing System doesn’t allow a shooter to fire as fast as with a gun equipped with a bump stock, but it can still double a semiautomatic’s rate of fire. It works by shooting twice for each full cycle of the trigger: once when the shooter pulls the trigger, and again when it is released. 

Where sold: Franklin Armory’s trigger system is sold by Optics Planet and Botach.com, though the latter store is out of stock. Online auction behemoth eBay also allows the sale of binary firing systems, even though it has banned bump stocks for years.