Those that are really interested in this should read up on Len Savage (Historic Ams LLC) and Bill Akins (Akins Accelerator).
Big picture take away - BATFE is part of DOJ - what is the general disposition of the DOJ with regard to civilian firearm ownership of this nature? Many of the BATFE's 'rulings' are based on opinion letters, they carry a caveat that such an opinion can change, based on interpretation of the law. These opinion letters are private matters, there isn't a repository for you to review. Thus it is easy to squash them b/c a change in the interpretation isn't likely to engender wide opposition.
There will be several that will point to the law - more than 1 round per single pull of the trigger - duly noted. In the abstract, and per the rule of law - you are 100%. That said, if you think we live in a period of the rule of law, I have news for you. The rule of man, has spent a good deal of time and effort to stamp out civilian ownership of certain types of weapons. While this product, bump fire stocks, etc may be within the letter of the law, they are in direct violation of the spirit of the law. You can bet that should these gain momentum, 'they' will get around to addressing the matter.