Legal thoughts on the Palm Pistol
The inventor of the Palm Pistol has added, not only the ATF approval letter, but Steve Halbrook's letter to ATF, which goes into much more legal detail. (It's all one pdf file, Steve's letter follows that of the ATF). The key argument appears to be that a "pistol" has a "grip" (whereas 19th century palm pistols did not) that is at an angle to the bore (which cane guns, etc. do not satisfy). It's all pretty arbitrary, of course -- but then the pistol vs. "any other weapon" distinction has always been. And AOW only got into NFA because of a quirk in how the statute was drafted and then amended ... it's not like the gangsters of 1934 were running around shooting at each other with palm guns from the 1890s, or guns built into a gentleman's walking cane.