I don't know of anyone manufacturing under the (relatively) new Firearms Freedom Act. I know I wouldn't risk ending up in some federal prison camp just to save a few bucks. There will be a "test" case at some point and that guinea pig will need the financial support to hire a not only a good criminal defense attorney but also a lawyer who litigates constitutional law. The associates working under such attorneys probably bill $200 an hour; How much was that tax stamp again?
One of the issues will include whether or not the materials used have/had an interstate nexus, i.e. was the steel/titanium used in the can a "product" of South Dakota. But of course, it doesn't just end there. There are plenty of arguments that the Gov't will have relating to the National Firearms Act and other legislation that pre-dates South Dakota's "FFA".
I love the spirit of our legislation but I'm not in the business of poking tigers or martyrdom.