I do believe there was only two of the Pancor Jackhammers ever built, one got a cameo in the movie Totall Recall. Also the HK CAWS was a select fire shotgun, IIRC.
The drawbacks to the idea is the bulk of the ammunition and typical range. For CQC only this would be an interesting firearm, if the operator practices fire discipline, for those drum magazines look very bulky to carry many of at once, no idea on the capacity of the standard looking magazines you see so briefly.
The Saiga is a nice semi-auto shotgun, wish I had picked one up years ago.
BTW, the company I referred to earlier, (it was a real outfit, no idea if they are still around, don't care enough to Google it), also made an "armor peircing" 12 guage slug. I also do not play those video games - hell, my first computer was a Commodore Vic -20, for crying out loud, I'm a little old for that stuff.
SPAS-12? No, the SPAS I would want would be the SPAS 15...