As a practical matter this can be easily avoided. 1. Outlaw possession of certain types/quantities of guns, ammo and whatnot, with draconian financial and criminal penalties. 2. Provide an 'amnesty period' during which owners can surrender the items in question without penalty, and (perhaps) a small financial reward.
No need to go around breaking down doors, though cops are generally not shy about doing that and, if indicated, showing up at your door with all sorts of military hardware, robots, helicopters, armoured vehicles &such. Good luck winning a shootout, unless martyrdom is your idea of "winning".
We don't have a window into this guy's mind and who knows what he was thinking, but I think it unlikely he'd have had a head count clocking up as he was spraying bullets. Given he was unencumbered by financial burdens, wouldn't the easiest way to execute this atrocity be to buy a missile laden jet and carpet bomb the stadium? Or are jets not covered by the 2nd amendment? From his point of view, at least he would have been able to get away. Perhaps he derived some perverse pleasure from being there first hand. :( I wonder how come it's guns that have become embedded in the subconscious as the armament of choice? Presumably, it's the portability and concealment properties that people like, and the bigger firearms have become more prevalent on the back of smaller arms becoming commonplace - essentially it's a penis measuring contest. Now, if penises had been the weapon of choice way back when, imagine the how the world would be today... I suspect Throb may well be the commander of the universe by now.