I want you, for just a moment, to forget guns exist.  I want you to look at a product as just a product. 

Let's say the head of the Food and Drug Administration* said, "Alcohol affects the brain exactly the same way as cocaine.  Therefore, we are declaring alcohol to be a narcotic.  Narcotics are illegal, therefore, all alcohol and all equipment to produce alcohol are contraband and must be destroyed immediately.  No compensation will be paid because narcotics are illegal."

Let's note: 

1) The definition is blatantly false.

2) The definition contradicts long standing legal definitions.

3) No legislative process exists. This is a fiat declaration by a bureaucrat.

4) Failure to compensate for a legal product taken for public use ("Safety") violates the 5th Amendment requiring fair compensation.

5) Most importantly, this opens the floodgates for ANY bureaucrat to declare ANYTHING illegal.

Remember those handful of conservatives who've said, "Declare Islam not a religion but a political movement and terror front"?

If a bureaucrat has the authority to state that AND ENFORCE IT, there is no Republic. Literally any cabinet head, or possibly lower, can declare outlawry, steal property, seize anything, without even the pretense that an existing law was broken. Law will be whatever they say it is, any day of the week. Any religion can be illegal or mandatory. Anything can be contraband or mandatory. The rule of law simply fails to exist. If this doesn't terrify you, I guess you can go now. Good luck. There's nothing I can do when they quite literally do come to put you in those camps you fear, which just became a solid reality.

~~~

Now, moving back to the relevance of this thought experiment. You're probably very gleeful over the AG's ban on "bump stocks."  

You were probably unaware of bumpfire/slidefire stocks until they were used in the Vegas shooting. In fact, though, they've been in existence since 2006, approved by BATFE as "not a firearm." This was reviewed in 2010 under the 0bama administration, and confirmed that they were "not a firearm." You probably like and supported 0bama, and he found no reason to try to restrict bumpfire stocks.

You've probably heard a lot of hysteria about them, and have no idea how one actually works, or, more accurately, how a person operates it, since it doesn't do anything by itself.

The definition Sessions is using is completely false and inaccurate. It's literally as inaccurate as saying "Alcohol is a narcotic." The definition given is that it "Harnesses the recoil energy of the weapon to function." Which is provably false. Put a firearm equipped with a bumpfire stock on a bench, pull the trigger, it will fire a single shot only. One. Then it will do nothing.

The second claim is that, just like a machine gun, it allows firing multiple rounds with a single function of the trigger.  This is also provably false.  Again: Place it on a bench, pull the trigger, it fires one shot.  The trigger must be released before it can shoot again.

This cannot possibly be defined as a "machine gun" that fires more than one shot per pull of the trigger.  Yet, that is what Sessions has done, via a false statement--a lie.

The manufacture of machine guns has been illegal since 1986. Real ones command a high price. The finding of the Firearm Technology Branch was not only that these devices were not machine guns, but that in fact, they are not even firearms.

Sessions has falsely declared it to be a machine gun, and therefore contraband ex post facto, with absolutely no compensation for the "contraband," in complete contradiction to the experts' findings.

ATFE estimates the device and related industry are worth $200 million, which is tiny in business terms. However, every owner, seller, maker has money invested that is being stolen from them. No due process.  No legislative process.

Look above again. This decision is precedent for ANY Cabinet head to declare anything...or anyone, illegal, and subject to theft or imprisonment with no process.

If you want to work on legislation that bans these devices, with an accurate description, and a justification, I will certainly fight it all the way to SCOTUS under the 2nd Amendment. That is why we have a legislature and courts.  But if you support bureaucratic fiat, you are signing your own eventual death warrant, and there's nothing I can do to help you. I wish I were being dramatic, but I am not.

* And consider this. It's now becoming illegal to treat yourself for diarrhea: 
https://www.medscape.com/viewarticle/891968