First you might try a 12" barrel insert and no baffles. Shouldn't need a tax stamp for that, and you can work on the extractor/ejector. The rest of the (24"?) shotgun barrel will be an expansion chamber; not sure if it needs to be vented or not.
OTOH, the shotgun barrel might make a nice resonator and you can screw a little horn on the end like Sledge Hammer's "loudener"