Couple of thoughts.
1. Strong case can be made that ATF never had any business evaluating accessories, because they are not firearms.
2. But now what will happen? There would seem to be two forces in play:
— without approval letters, vendors and purchasers could be afraid to buy something new
— but without disapproval letters on new designs, a lot more accessories might get purchased. If enough are purchased, there won’t be any option to ban.
So now, what if a NEW bumpstock design enters the market by some other name?
The proposed rule would seem to cover anything that functions similarly to a bump stock, regardless of what it is called. Here's a copy of the notice of proposed rule making (the proposed rule itself is discussed beginning on the 6th page of the document.):
https://www.gpo.gov/fdsys/pkg/FR-2018-03-29/pdf/2018-06292.pdf.
With regard to evaluating accessories: This is something the ATF has been doing to help out manufacturers so that they can get a ruling that their AR "pistol" is not actually a short-barrel rifle before the thing is sold. No one is required to submit anything before selling a firearm. It's just that the line between "pistols" and "rifles" has become so arbitrary that AR pistol manufacturers understandably want to get the ATF's blessing before selling, lest they unwittingly sell something that turns out to be an NFA Class III weapon.
For example, we know today that an AR-15-pattern firearm with a 10" barrel is a rifle, and thus a "short barrel rifle" under the NFA, if it has a regular stock because it is designed to be fired from the shoulder. But, under recent ATF rulings, if it is fitted with a "brace," it is a pistol that can be sold just like any other handgun. The reasoning is that it is not "designed" to be fired from the shoulder, even though the "brace" looks suspiciously like a stock and even though virtually no one who buys an AR pistol uses the "brace" as anything other than a stock.
But add an angled foregrip to the picture. Is it still a pistol? Because it's hard to see how a firearm with an angled foregrip is not designed to be fired from the shoulder like a rifle.
With all this confusion, the ATF really should be ruling on these before people potentially become criminals by buying them.
Or, better yet, Congress should abolish the pointless restriction on SBRs that causes all of this confusion in the first place, which would make the ATF's job a whole lot easier. That's why I suggested that, counter-intuitively, the ATF might be supportive of taking SBRs out of the NFA. In a sane world, this would be uncontroversial even among supporters of gun control because the SBR restriction complicates enforcement and provides zero demonstrable public safety benefit. But, of course, the same could be said of any "assault weapons ban," and so we know we don't live in a sane world.