Or the previous white guy with a gun? Or the one before that? The problem is not "mentally challenged" people have access to guns, the problem is people have access to guns that are designed to kill large numbers of people.

And portions of problems can be fixed. There need not be one solution that fixes every aspect of every problem at once.

from gun purchases would be effective is what?

And they can be fixed individually, and even partially.

A favorite ploy of gun rights people is to insist that any potential solution fix all aspects of all problems at once. It isn't worth doing unless it fixes absolutely everything (mostly because they appear to want to do nothing).

"Will it stop ALL mass murders? No? No good, can't be done." That's the line of people who are either afraid that their firearms are in danger or profit from sales of firearms. These people will argue to death against anything that could possibly restrict firearms in any manner. No point in arguing against them, so I generally avoid it.

It wasn’t actually a measure that block people who were mentally ill. It was one that blocked people who had another person designated their representative payee for Social Security no matter why they did so, with no due process.

And in this case there is zero evidence that it would have affected this at all, so bringing it up in relation to it is dishonest nonsense.