Is there a legal reason why it isn't at least a six-shooter?
If you sbr/sbs it, di you have to pay two tax stamps?
^from the ATF NFA FAQ page. A firearm that meets the definition of a SBR consists of a rifle that has a barrel less than 16 inches in length. A SBS consists of a shotgun that has a barrel less than 18 inches in length. The serialized receiver is recorded for registration in the National Firearms Registration and Transfer Record (NFRTR).
If they would chamber it in .38/.357, they would sell a lot more of them.