The problem isn’t so much the law going forward (which will be a delightful debate, I’m sure) as the fact that there are currently about 275 million firearms in the United States.
Aaaaaaaaand, more to the point,
I’m going to bet that the majority of them are owned by the ”pry ‘em from my cold dead hands” folks* who have been freaking out about losing their guns since loooooong before anyone seemed ready to address the issue.
So. To restrict sales of new firearms would be a nice start, but there is no shortage of firearms currently in existence because America so saturated the market in the first place. That is a problem.