I see this argument all the time, is there any real proof?
I mean, it's easier now to legit buy a gun than use the black market. If every gun was tagged, ammo was scarce, and it became a felony to lose a gun without reporting it, you really think that suddenly the black market (whose main resource for aquiring guns), will just explode with psychos left and right buying firearms?
That a heat of the moment crazy dude will go to some gang and pay them money and that isn't any different or a deterrent at all when compared to walking into walmart?
Nobody realistically is asking to make guns impossible to get, but making them harder to get adds a barrier of safety. It prevents heat of the moment attacks, it gives psychos a period to think things over and it slows down the whole process.
Unless you'd like to argue a dude that wakes up and says "I am going to slaughter 2 dozen kids today" walks into KMart, buys a gun and does his deed is not different at all from that same dude waking up, and then having to find a local gang, manage to gain their trust enough to buy a gun, and then do the deed? One requires a hell of a lot more work than the other and it gives the would be killer more time to reconsider, to calm down, to change, or to just be spooked into not going it.