No, of course they don't know who has them and they don't need to waste time or money trying to track down purchase records. All they need to do is pass the law and wait for it to be broken. If you get pulled over with a bumpstock in your trunk, you get pinched. If they come to your house for a fire and find a bumpstock you get pinched. Some cop doing a "wellness" check during an emergency sees your bumpstock, you get pinched. If you get "Swatted" as a practical joke by some punk kid and they find your bumpstock, you get pinched. And 500 other scenarios I could dream up. The more BS laws they pass the more criminals in waiting there are, and that's the goal in MA. It only takes 1 OOPS to become a PP. My advice is, don't risk it, get some friends in NH or at the very least a storage unit.