It'd be all in the way we went about it. I'm thinking of my grandkids, not next month or next year. You'd start with a ban on manufacture of the prohibited stuff, a prohibition on new sale of those items still in stores, and an immediate buyback of all unsold inventory from dealers/stores, etc. Then a very very extended (several years at least) buy-back or $ for $ tax credit period for the literally tons and tons and millions and millions of pieces of soon-to-be-illegal-but-not-yet stuff already in private hands. Then an amnesty period before the criminal penalties become effective.

I don't think there's much racial disparity in the % of ownership of clip-loading semi-autos (I'm including handguns in this), and so wouldn't expect a racial conviction disparity other than the general disparity that already exists (50% of pot arrests are minorities when pot use % is virtually the same for whites and minorites for example). If semi-auto long guns only (not a compromise I'd want), I'd bet there are more white owners of those. And the years-long progression from buy-back to criminalization would give everybody who wants it a chance to avoid penalties, so I don't see a huge increase in the prison population resulting as long as a very extended timeline was a part of it.