Hi Wigeon Jim,
Quote "Most will say never use plastic wads in these guns, I have used my one for over 15 years and have fired 1000s of plastic wads through it with no problems, keep any 8 bore data at least 2 grains light for these guns and use felt wads if you dont want to risk plastic, Many have bulged just before the chokes, Jim."
This gun has only got 28 1/2" barrel so that could of happen to this one, had my gunsmith check it out before and he said it was just under 1/2 choke.
Any chance of details of your loads and what type of plastic wads were you using.
Dogfox1