Stephen King on US gun contols.

[MH]

Then the obvious issue, a person that wants to do a mass shooting can't get a gun legally, they'll just find one illegally.

Maybe he will maybe not.
Finding gun illegally may involve risk and getting caught in the processes.
Some may succeed some may not.
There is also the issue which guns are suitable for self defence and which are for mass shooting.
You can say that explosives could be sold in walmart just because if someone wants to blow up empire state building he will anyway.