You start by enforcing the laws at point of sale, manufacturing and imports.
Thank you for engaging with the questions I asked.
While the bolded works to prevent the future sales of semi-automatic weapons from licensed dealers, how does one reduce weapons already owned from circulating? There are enough guns in the U.S for almost everyone to have two, and plenty of people who have hoarded them.
You mention that there can be hefty fines when somebody is found with a weapon without a license. But many people with these particular weapons live in Second Amendment sanctuaries.The likelihood that they would even be reported and/or there is evidence that they own the weapon (since there are no registries) is slim if they live in these counties/states. Buy-back programs would have to be very generous, especially when the price of the weapon suddenly increases if there is no new production. New York for example had a buy-back program with very low compliance.
The discussion surrounding race isn't to make an excuse, but to bring it to the forefront that enacting even more strong gun-law criminalization will lead to more inequalities between races. It is only a separate issue if the race issue is addressed before or concurrently to the enactment of the laws. Addressing the race issue addresses many of these mass-shootings at the source anyway. Advocates of gun control as the solution, should be even stronger advocates against white-supremacy because eliminating white-supremacy is the only mechanism in which gun laws can be equitably applied.
Having said all of that, I do support a licensing system. The sort of licensing system that is found in the Czech Republic would probably be quite an easy sell. Alternatively, one of the best ideas for a licensing system I've heard is to have multiple different classes of fire-arms based on if they are rim-fire or center-fire, concealable vs. non-concealable, etc, and to couple the license with a nationwide carry permit to act as an incentive for gun owners to become licensed so that they can carry seamlessly across states. It isn't clear to me that this really solves the problem of there being hundreds of millions of guns already in circulation though, although it does solve other problems -- like educating gun owners on safe-storage, accident prevention, theft prevention, etc.