If the Harris/Biden administration also has the Senate and House, they'll try to add everything semi-automatic with a detachable magazine to the NFA registry, with a limited grace period to get your tax stamp for each item you own. (And don't expect the tax stamp to stay at "only" $200.) They probably won't go for outright confiscation . . . at first. But look for a lot of problems at the state level in places that already restrict current NFA items like suppressors and non-C&R full autos.
Without both houses, most of the mischief will be limited to import-related items, since the Executive Branch has broad powers there . . . so long as the GOP stands firm.