Manufacture have to apply with the ATF their designs on new type firearms. I remember reading the documents from manufacture that started the "Bump Stock" and went for ATF approval. There was also a "Trigger" that came out around the same time. The manufacture had a Patent on it and originally was getting approval from the ATF The "Trigger" had a reset that was almost like "Full Auto" but still had to be fired for each shot. It was a lighting fast reset. Don't know if that company ever made a go of it. I'll try to do some research on it.
The government writes up rules and then manufactures look for the Legal wording in the Rules to make workarounds.
Look at the "BumpStock" compared to a Full Auto. A few modifications on the trigger guard and stock instead of in the trigger housing turn it into Full Auto.
Personally I think if the Manufactures are within the Rules and then The ATF Approves it . The firearm should be good to go. Anyone looking at an AR Pistol with an Arm Brace can clearly see that if I put my AR SBR next to it that they look and function almost the same.
I also can't see why there is a classification of an SBR other than the government getting $200 TAX on it. What is the difference between a 17" barrel and 15" with a stock other than the $200 and a background check.
Also the NFA National Firearms Act in 1934 started the ban on civilian owning a Machine Gun (Full Auto) then in May 19, 1986 prohibited the buying and selling of Full Auto.
Criminals can have Full Auto, but legal citizens can't (unless you have special licenses). It is all about politics and $$$$$.
Not true about full autos
As far as federal laws -- full autos are not illegal to own, buy, or sell-- you just need to buy a tax stamp for them registering them and the owner in the nfa database --- what is illegal is the manufacturing of a full auto after 1986 to sell to civilians.
Some states have made full auto illegal but not at the federal level.
Lots of people think that machine guns, suppressors, etc are illegal-- you can have them- you just have to register them and pay the tax stamp (gotta pass a bgc just like any other firearm from a dealer- nothing special about it really)
The process is exactly like buying a suppressor.
NFA items include : machine gun/full auto devices, short barrel rifle, short barrel shotgun, suppressors, "any other weapons", and even DD's -- federal law says you can own them-- just have to go through the atf/pay the fee/ bgc/ fingerprints/ notify local authorities/ get tax stamp before transfers can be done .
Read rhe rules, know the laws, knowledge is power--- in today's world the pen is mightier than the sword