S&B brass tends to be pretty brittle in my experience. I don't know that I would use it for any fireforming. I can't offer any guidance on this case. I have no experience with it. A Martini in it does sound cool, though!

interesting cartridge. I'd like to hear more when he gets it up and running. I assume this has to be in the large frame Martini?

It should be very close. The rim is smaller but that should not matter. The body of the case is similar in diameter and length to .30-30.

I know that you can form the Herrett cartridges, maybe the smaller JDJ cartridges from either case.

Once upon a time, Norma made 22 HP brass. Dunno if they still do. Much better brass than S&B.

Ken Waters mentioned in some article, that these little 22 hotrods had severe case loss on forming. I would try to locate some Norma and anneal. Is he fireforming with a full charge & bullet or fast powder vertical?

When I bought my 219 Donaldson (back in the 60s) the gunsmith made me an intermediate forming die to get from 30-30 case diameter down to about 24 - 26 caliber in two steps before running it into 219 DW die.

Same would be useful here as well as annealing the case and GOOD case lube. NOT too much lube though.