A Florida man invented and patented a device that would automate the bumping procedure on a trigger. The effect would be very similar to auto fire.
Bump firing is performed by letting a semiautomatic gun's recoil in effect pull the trigger. Initially, BATF allowed the invention which attaches to the trigger guard, because it did not meet the definition of a machine gun. Then, they renegged their decision effectively bankrupting the inventor.
Here's a youtube link to how well the invention works on a 22 rifle.
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