SayUncle reports that there is a 19 round drum out for the Saiga 12 now and notes that this might render them classified as Destructive Devices by the ATF. It’s happened before with other shotguns.
I currently own a .410 Saiga and once owned the 20 gauge variant but I gave that away to a buddy. They’re nifty little weapons, and a pretty good value with them being, at least at the time I was buying them, $200 for the .410, $230 for the 20 gaue, and $300 for the 12 gauge.
If I had the cash on hand to buy the 12 gauge and one of those drums I’d be on that like white on rice.