Something fundamental to consider here is the difference between a LAW and regulatory power given an agency under that law.
It is pretty unusual to see a law written so specifically as our National Firearms Act's rules on barrel length for rifles or shotguns. There they wrote two specific numbers and said "this is the law." For lots of other things -- such as silencers as a great example -- the law outlaws, limits, or regulates something but then leaves to a regulatory agency the duty to define, decide, and enforce what is or isn't covered by that law.
So, the law says that a shoulder fired, rifled-barrel firearm is a rifle, and if it has a barrel ANY AMOUNT less than 16" then it is an NFA-regulated firearm. Period. No leeway. And that's ok, in so far as the items in question are pretty clearly a binary "IS/ISN'T" thing. (More or less...but if you add a muzzle break...?)
Silencers are regulated too. But what is a silencer? Any tube on the front of a gun? Anything at all that makes a gun quieter? How much quieter? Etc. The ATF has to interpret that law and decide that yes it's any amount of reduction, but no just a bare tube doesn't meet the spirit of the restriction, and, well...no even if you make the barrel 40" long JUST so it will be quieter, we aren't going to call that a silencer, etc., etc., and other details like that.
This is an inherent function of trying to write laws that place concrete limits on all the possible permutations of human ingenuity and design. As we see so often with the NFA (again), Congress writes a concrete law that says X is regulated and years later someone invents something that is not quite just like X but works just like X and without an agency with some ability to make determinations and regulations, the law wouldn't be flexible enough to deal with that. (I'm talking to YOU, Mr. Sig arm brace and Mr. 3MR trigger!
)
So, no, the BATFE probably doesn't at all want to publish what will be seen as an eternally binding document defining EXACTLY what ISN'T a firearm, because it will have to be either so restrictive as to cause great disruption of the
status quo or tomorrow someone will have found a work-around that meets the last exacting letter of that determination document but is obviously and completely at odds with the whole concept being enforced.
Now...I don't know that I really have a problem with the BATFE being backed into a corner that way and forced to write a technical code that can be pointed to as
carte blanche to do anything that comes right up to the knife-edged line of the rule.