I have that same special issue hunting tag on my truck

I havent used my mossburg 500 using slugs in 15 + yrs but when i did shoot foster type slugs deer hunting etci never had a problem with it.(think it had the long trap vent rib/factory type barrel on it backthen.

yep They seem to have a variety of aftermarket barrels at good prices- slug barrels etc& a blackpowder barrel.

Id agree/go with the obstructed barrelpossibilitys