My FIL hunts duck with a 10 gauge. It is subsonic and the loudest gun I’ve ever heard. I can hear him at pond almost a mile from my house shooting duck.
Your FIL's duck loads will exit the muzzle supersonic but will be subsonic by the time they get to the ducks.
http://guide.sportsmansguide.com/ballistic-chart/wballistics_192.html