Slide Fire stocks have no serial numbers and are plastic. Good luck tracking them.
The problem is, if they catch you with one, you’re now in for a world of hurt. I’ve got no use for a bump stock since I’m in the city—couldn’t use it at the range, wouldn’t use it for home defense. If I was in the country, it’d be kind of fun. But, not worth the risk now.
If they were purchased using a credit card or by mail order, the Feds might start making visits.