The Dayton and El Paso shootings have spurred familiar calls for more gun control, and by all means let’s have a debate. But the focus should be on denying weapons to the potential killers rather than on gun laws that may be politically satisfying but won’t make much difference.

Start with the calls for more “background checks,” which implies none now exist. Yet nearly all gun purchasers today have their backgrounds checked on the spot via the National Instant Criminal Background Check System (NICS). Most mass shooters obtained...