Pact makes a strap on shoulder pad for helping take the hurt out of heavy kicking rifles. I split the stock with a lead sled on a 460 Weatherby mag. The recoil of 3" buckshot is about like a 300 Weatherby Mag.
Don't be surprised if #4 Buckshot out patterns Dead Coyote, it did in my two shotguns.
My Hastings turkey choke produces 20" patterns at 40 yards with #4 Buck, my hand loads and Federal Copper Plated, buffered #4 buck.
The gentleman that suggested wearing a life vest to take some of the kick out gave the best suggestion that I have ever heard.
My wife had a pad that she places under her knees when she is knealing down working in her flower garden, the material in the pad I believe is sorbthane. I placed the butt of my gunstock on the pad and traced and cut out a perfect little pad, next I doubled the pad, taped it on the gun with duct tape...it helped a lot.
Good luck!