Midway sells 32 gauge shells. Considering how oddball it is, I thought the price was surprisingly reasonable.
If I remember Ed Hubel's 12gaFH thread on Accurate Reloading correctly, he got a ruling from the ATF on his .720 caliber rifles. The ATF said that even if the barrel was rifled and he was shooting blown-out BMG in it, if it would still accept a standard shotshell, it was still a shotgun as far as they were concerned.
Of course, shotshells have tiny rims, and with enough extractor hook travel, there's no reason you couldn't shoot a rimless brass shell, as long as the rifle could still shoot a standard shotshell. At least, that's how I understood the thread. It's something like 65 pages long now...
I have a 4140 bar cut up to make bolts from. Depending on how the feed issues work out, I could always make a Saiga-length bolt if I had to. I'd like to do it without going that far, if possible, so others could duplicate it without a lathe and heat treating a bolt.