Increased gun sales are motivated by fear of confiscation. Mine! Mine!Mine! cries the bitter clingers when threatened. 
The earnings of Sturm Ruger fell 50% when tRump was elected and the fear of the Muslim black president was gone.  
There's no denying those mass shootings increases the value of gun stocks as the paranoia creeps back in. The easiest solution would be to register and tag ammo and limit the amount you can buy.  The "militia " would have to go to an armory just like the military to receive their allotted ammo.