Hushpower in England has been doing shotgun suppressors for several years. They use a drilled barrel and a couple of baffles out front as the load leaves the choke.
They will sell you (in England) a suppressed 20 ga Over-Under on a Spanish Fabarms action complete for 830 Pounds = $1402 today. No transfer tax, if you go to the trouble to get even a shotgun certificate, a silencer is just being neighborly.
A Mossberg 500 with Hushpower barrel and standard is 499 Pounds = $843.
A detachable "moderator" is only 160 pounds = $270.