What kind of a choke does it have? I have on old Mossberg 500 with a short cylinder bore and I can only describe the recoil with 2 3/4" 1 Oz slugs as brutal. I filled the stock bolthole with #2 shot and that helped some. I finally went to a rifled cantilevered barrel and the extra weight and length made it 'shootable". I can only imagine what the recoil from a 3 1/2 " slug might be. If your gun has choke tubes you might try a rifled one. I have one on my Rem 11/87 and it shoots just as well as the Mossberg with the fully rifled barrel. I just shoot Foster type slugs through mine but both guns will shoot 3-inch groups from the bench at 50 yards. If you find a 3 12" slug let us know what it feels like to shoot one,
RJ