Quoted:
That being said, you can't turn double barreled shotgun into a semi-auto. For a shotgun to be semi-auto, it has to start out that way.
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A common and understandable misperception. Fact is, guns are just mechanical devices. If one has sufficient skills, resources and motivation, it is possible to fashion and alter parts and assemblies to convert virtually any gun from any action type to any other action type. For example, Winchester 94 leverguns can serve as drum-fed full auto smgs with a bit of work. And bolt actions can also go full auto with parts such as the Pederson device.
A standard pump shotgun could be modified by a suitably trained person to become a magfed semiauto. The Sidewinder mag conversion could handle the feed conversion and an appropriate spring and piston assembly could be fashioned to convert the gun to semiauto operation. A new fixed forearm would also be useful.
Though in the case of the prosecutor's comments, ignorance and poor judgement are in my view the more likely explanation.