In the episode the gun was made for Benelli as a showpiece(demonstration gun), they awarded it to their sponsored three gun shootist. With the proper licensing it is possible to convert a semi to full auto for demos. I've heard of similar types of work done to glocks to mimic the glock 18 for demonstration only for LE. The gun in the episode is probably registered to the Benelli corporation and has the shooter listed as caretaker or some legalese on a paper somewhere. If they built and filmed it, it's legal.