Well, we definitely know who the gun-nuts and the wannabe terrorists will vote for -- if they can vote
It is easier to buy an assault rifle here that to buy a dog license. You don't even have to renew your long gun license. If you had to pay a $10 renewal fee and file a license report -- a lot of stolen guns might be accounted for -- [x] firearm was stolen -- no fee. At least we could track such things? Oh Yeah, $500 fine and a federal misdemeanor for filing a false report. This goes toward firearms illegally possessed by criminals mainly.
Firearms are a regulated industry and most regulated items have license fees. Make the license fee a tax-credit item but you have to pay to stay legal
Of course, that is a year (annually) after the fact (the sale). This won't directly help mass shootings as they are not planed that far in advance -- they are usually impulse actions -- the straw that broke the camel's back (no pun intended).