First off any law that restricts an individual from having any handheld firearm that he desires to possess without being individually determined through due process to not be trusted with it is facially unconstitutional. This is per the actual wording of the second amendment and the supreme Court Bruen decision.
Also when one considers at the time of the founding the militia was the primary means of military defense of the United States, standing armies being dangerous to Liberty, the argument that the founding fathers would never allow people to have machine guns is patently ludicrous. That's like saying the Army shouldn't be allowed to have them because at that time they were the army.
Why on Earth would they handicap themselves when they were betting the very existence of the United States on civilians owning guns that were at least on par with the ones they would be facing? In addition privately owned warships and cannons abounded, the United States basically legalizing piracy as long as you had a government document authorizing it on their behalf and they bought or borrowed many a Cannon also.
They were also aware of fast firing guns of the time because people tried to sell them to the government. No regulation, that didn't happen until 1934 long after the era Bruen allows for contemporary laws to be considered. The actual ban on new machine guns is actually much newer, 1986. As far as I know every supreme Court decision and most lower Court decisions on machine guns use the outdated tests and legal theories that predate Heller and everything that followed.
Second, laws are written to prohibit something, not to allow it. If the law is silent on something, it is explicitly allowed. The definition of machine gun is specific, you move the trigger and hold it there and if it continues to fire more than one shot it is a machine gun. Full stop, absolutely nothing else is a machine gun no matter what mechanism it uses or method as long as the trigger, defined as the part that initiates the firing sequence, has to be moved for each shot. That is not a so-called loophole, that is the law being silent on any other type of fast firing mechanism. And thus explicitly allowed.
Third, the only possible way machine guns or other NFA items can be considered to not be covered by the second amendment is that old dangerous and unusual or not in common use thing brought up by Heller. It is precisely because the government acted early to curb the number of these particular firearms that they are not in common use and are considered unusual. Look at the popularity of these devices these days, if you could go into any gun store and buy an M16 for $5 more to cover the extra part and walk out with it how many AR-15s you think they would sell? They have a selector on them, if you don't want to or can't afford to blow away a few hundred dollars in a sitting you don't have to. They would be common as dirt considering the number of AR-15 sold in the United States. It is that artificial scarcity caused by the previous laws that might allow machine guns to be considered not protected by the second amendment.
And fourth it is these not a machine gun devices' popularity without any meaningful government regulation yet that offers a glimmer of hope to get machine guns off of the NFA or at least get rid of the ban on new ones. Rare Breed alone sold over 100,000 before ATF told them to stop. How many bump stocks existed prior to the changing of the regulation? Supreme Court has said bump stocks are not machine guns, precisely because they require a separate movement of the trigger for each shot.
This also directly applies to FRTs, they require the same thing. Bump stocks came back on the market over a year ago, FRTs by whatever mechanism they use are extremely popular, how many makers are there these days? It would not surprise me if by the end of the year there are close to a million if not more legal not a machine guns out there. By the time any federal legislation could get passed and signed by the president there will be a metric shit ton of them out there. Not unusual, in common use, and presumptively constitutionally protected by the second amendment.
They are going to have a major uphill battle trying to get them banned with a law that will stick. The absolute worst thing they could do from their viewpoint is not create a separate category of NFA for these alone, if they simply change the definition of machine gun to include these they have now added a million or more legally defined machine guns to those out there, making machine guns the same common and not unusual guns. There goes any NFA on them as well.
The best chance for overturning the NFA in my lifetime is right now. Congress has already reduced the tax on everything but machine guns and DD's to zero. They tried to eliminate the registration as well but were blocked by procedural roadblocks. We have at least a somewhat gun friendly supreme Court lineup right now, at least one that looks to the original laws instead of making things up as they go along like many of the lower courts do. If we can get the right case with a sympathetic guy on our side of the argument there's a very good chance things will go back to the way they were pre-1934.
And all this hinges on making something sell so well it is in common use. Instead of complaining about the existence of them which is not going to go away you should be buying as many as you can afford.
As for not wanting our criminals and urban Utes to have machine guns, with the appearance of the Glock switch and the easy manufacture of it they are as common as dirt in places like Chicago. They are already illegal and like the flow of drugs across the border they can't stop it either. Rifles which are the main focus of FRTs are used in so few crimes it is statistically insignificant. You're more likely to get stomped to death by a pair of heavy boots, perhaps we should start regulating who can buy work boots?