Automatic weapons were effectively banned in the U.S with the Firearm Owners Protection Act. In order to get one you need to go through a 6 month FBI background check and pay for a $200 tax stamp. All automatic weapons made after 1986 are illegal for civilian purchase, and everything else that had been grandfathered from before then is very expensive ($20,000 - $30,000.) Bump stocks, which convert semi-automatic weapons into fully-automatic weapons were banned via ATF regulation and the courts have held the ban so far. But the ban is hard to enforce because bump stocks can be made very easily and cheaply.
The difficulty with banning semi-automatic weapons is that they are the most common weapon-category produced by manufacturers and the Supreme Court in D.C vs. Heller decided that any weapon in common-use is protected by the Second Amendment. Plus there are about 300 million semi-automatic guns in the U.S, and rounding them all up would cause considerable problems given the unconsented policing-landscape of the U.S and the strong ability for organized crime to develop around their restriction (like with drugs and alcohol prohibition.) If semi-automatic weapons were banned when there were few of them, like what was done with automatic weapons, it would be pretty easy to enforce, but since there are so many, the problems with enforcement arise. You might observe a situation where white people who live in rural areas where police don't want to enforce gun laws are affected very little, while BIPOC people who live in jurisdictions where police are more committed (mostly urban areas) in enforcing them feeling the brunt of these new laws. This of course leads to many other inequalities, and there is a widespread movement currently to move away from mass-incarceration, not create new crimes where there weren't formerly any that will almost certainly disproportionately harm non-white communities and persons.
What are some ideas for enforcing bans of semi-automatic weapons that would get rid of about 300 million guns, in a way that doesn't disproportionately harm marginalized people, soon enough before the next political cycle leads to the repealing of such a law?
You start by enforcing the laws at point of sale, manufacturing and imports.
Plus a voluntary surrender of weapons program, cash for weapons handed in.
A year later make it fully illegal to own semi automatic weapons without a special license. Hefty fines when found with an illegal weapon. If used in a crime, higher punishment.
Enforcing doesn't mean go house to house to search for weapons. But make it very clear through advertisements that you have x days left to surrender these weapons, after which hefty fines and higher sentencing come into effect when found with such a weapon.
Racist issues among the police are a different problem that needs to be addressed as well. You can't use one wrong as an excuse not to fix another wrong.