Swapping bump stocks for reciprocity or anything else may not be as easy as all that. If the stocks suddenly become illegal or heavily regulated, what do we do about the zillion or so already out there? A six month amnesty is about all I can think of and I'm also guessing that the compliance rate won't be any too high.
If fitting a bump stock makes an otherwise legal gun an illegal machine gun, again, what do we do about the ones already out there. Does turning in a bump stock provide proof of ownership of an illegal machine gun? If so does the 5th amendment excuse anyone from turning one in?
The bump stock was originally approved as an aid to the handicapped to assist them in shooting.* Banning the stocks would then be a violation of the Americans With Disabilities act. Now what?
*Whoever suggested this deserves the Brass Balls Award for extreme chutzpah. Whoever bought off on that argument needs professional looking after. I understand it went down in the O'Bummer administration.