Hey,
I went to a gun show today. I asked about the folded stock length to two dealers of T/C stuff. Both didn't know of any regulations regarding minimum folded length. Furthermore, both have said that they have openly displayed 16 inch carbines with the folded stock, and have not had so much as a second look from ATF agents visiting their tables.
I know this isn't matter-of-fact from a legal perspective. But hopefully, it will be somewhat more reassuring.
B. Noz.