Hmmm... Not much of a demand for a 80% version of a shotgun.
What makes the rest of the 80% world practical is the rest of the parts you don't have to make, like barrels and fire-control parts being available. With a shotgun, the barrel(s) are the heart of the firearm. Only a few are readily available to work with.
For the purist, a receiver lump for a break-action double used to be something you could buy (been a while, but I think Track of the Wolf and Dixie used to sell them.) The Pedersoli kit that Alpa Chino referenced is a very refined kit. There used to be kits from Connecticut Valley Arms (CVA) that were cruder, but along the same lines. Yes, they are black powder. If you picked up a shot-shell in the mid 1800's it looks a lot like a modern shotshell...but it was loaded with black powder. The break-action shotgun if one of those few arms that pre-date modern propellants, being just a outgrowth of the muzzle loading percussion gun. Building one doesn't lend itself to being an 80%... it is all hand work.
Pump Guns... I don't see the value of making an 80%. You can get a used Mossberg 500 dirt cheap. Maybe a used Rem 870 or Ithaca 37 for a little more coin. I've found many a 12ga 500 for less than the cost of a Polymer80 kit. But this is the best place to cut your teeth on. I learned stock fitting and barrel alterations on Rem 870's and Mossberg 500's. Did a lot of Tru-Choke installs, long forcing cones, and restocks on these kind of guns.
Semi-autos... yeah, I can see that happening. AR uppers that feed shotshells, that's a thing already. Hey, if they could modify the Mauser 98 to take two 12ga shotshells, why not take any modern auto loading rifle and shotgunize it. Curious to see how that fellows
