FWIW, a recent post mentioning a threaded muzzle reminds me of something I discovered way back in the 1980s with my old Topper shotgun:
I had acquired a 6" rifled chamber insert in .357 Magnum and when I shot it with a few .38 HBWC target loads I could barely hear the things go off. The extra volume of shotgun barrel past the end of the rifled insert acted as an expansion chamber, and from the shooter's perspective it sounded about like a .22 CB cap.
I changed jobs and sold my Topper soon after this discovery (went from growing almonds to shelving books in a public library), so this experiment went onto the back shelf.
I'm no lawyer but I think this might be a legal approach for coping with the silencer ban in California -- if there was enough accuracy, I was thinking about using it for shooting ground squirrels in an orchard next to a rural cemetery.
Rifled insert makers tend to flit in and out of business -- I just found this one in case someone else wants to give this a try:
http://www.gunadapters.com/12-gauge-to-45-colt-pathfinder/