When they regulated the the full autos in 1934 they used a tax. Everyone who had one had to pay a $200 dollar tax to keep the firearm. That was a lot of money in 1934.
Many of the leading Democrats want to do this with semi auto rifles today. I don't think it will be any kind of door to door confiscation. If you own a Browning P35 pistol with a shoulder stock you must buy a tax stamp to own it. That is one example under the SBR rules. The transfer stamp is a one time tax.
We don't know anything yet. Becareful about making moves this early in the fight. My concern at this time is the Republican Party is weak and running from the fire. If the Republicans fold we are in big trouble. The Democrats will move to a total Euro ban like England.