Citation for that?
From the various proposals I've read, the one that makes the most sense is requiring a background check with references, where at least 2 references have to sign an affidavit that they know you well and answer certain questions before your permit is approved. One of the reasons this has a much better chance of working is that A) a lot of these mass shooters were already suspicious as fukk long before they went off, but people just "didn't think they'd go that far" and B) they're the types who don't want anyone in their life to know what they are doing. I think those two factors would have a chilling effect on at least a portion of the shootings, not even counting the ones who wouldn't have passed the mental health/criminal background checks anyway if background checks were actually rigorously enforced.
Beyond mass shootings, that would also stop a decent number of crimes of passion and especially a lot of suicides. There was a great article in the Oregonian some years ago regarding how many suicides are committed by people who just recently bought the gun specifically for that. They could "try something else", but often people are reluctant to try a different method and things like "taking pills" have a much lower death rate.
It won’t work most mass shooters come from white conservative to moderate families. they will make excuses for odd behavior and co sign the person getting a gun