Yep, how many bumpstocks are there already out there in the wild. I don't think any have serial numbers on them. And I want to say I saw the first ones at a gun show in the mid- to late- 1990's (and thought "Meh, what a great way to waste ammo.")
So if they do move to make them NFA items, there's no way of knowing how may exist (maybe the makers have records of "We produced X per day/week/month.), nor who owns them. I bet there will be a run on large diameter PVC pipe and end caps.
Anyway, I also see that those are going to thrown under the bus. Even the R's are saying they have to be "regulated". The D's of course are going to try to overreach (Yes, I know. Shocking.) and go for the big prize, or at least more than just bumpstocks. If they didn't they wouldn't be D's and it will end in failure, because even they can't agree on what they want incrementally (Besides "All of Them").
I do hope that:
1. Putting bumpfire in the NFA inadvertently re-opens the NFA registry, however they could word it so that ONLY bumpfire type devices get added (and nothing else).
2. We get something in return. National reciprocity, HPA, something anything, at this point.
3. It could be very possible that eventually nothing will happen. Given that the legislation is all over the map, and various competing bills (many with "A Bridge Too Far" or Poison Pill provisions), there's a strong chance that nothing will pass, and the R's will take another "hit" for not being able to get anything done. Which would be a win in our "favor". The liberals and many D's are still dancing in the blood of the victims, and the heroes stories are just beginning to be told (people who sacrificed themselves to shield others, the Marine vet that "stole" a truck and made several trips shuttling wounded to the hospital while there was still shooting going on, etc.) Those will be the stories that slow the gun-grabbers roll.
4. So I think time is on our side. This is big story for now. However, soon the media will turn it's attention to the latest Trump Tweets and his other outrages. He's too big of a target for their hate and rage to go unnoticed for long enough to keep focused on LV. Which is a good thing that the media is like a PCP-addled Squirrel when it comes to remaining focused. 6 months to year there will be a small flurry of stories of "On the 6 month/1 year anniversary of LV, still no meaningful legislation on gun-control."