Really, we should encourage spree shooters to use fully automatic weapons. They'll run through most of their ammo without hitting the target.
I'm hoping there's a bit of sarcasm here, there's a significant difference between hitting what you're aiming for and just plain hitting something. Spree killers and the like rarely particularly care what they hit, just that something in the general direction gets hit. Adding more ammo to that isn't gonna help.
Plus given the number of them that wind up dead or in jail till they're dead, I doubt they have much issue with dropping what money they do have on the weapons and accessories they need. The north hollywood boys certainly showed what could be accomplished with a decent chunk and no concern for others.
The North Hollywood bank robbers spent a patently ridiculous amount of money on their weapons. I can't easily find sources for the cost back in the mid-90s, so I unfortunately have to use the higher prices today. But here's a rundown of what they had:
2 Norinco Type 56S rifles: $1000 each
Norinco Type 56S-1 rifle: $900
HK91 rifle: $2500
Bushmaster Dissipator rifle: $700
Beta C-Mag for Bushmaster: $265
Beretta 92FS INOX pistol: $800
2 Kevlar vests: $525 each
Metal trauma plate: $115
Total cost for equipment: $8330
This isn't including the cost of the magazines for the other weapons, the ammunition (they were previously arrested with over 1600 rounds of 7.62x39mm and 1200 rounds of 9mm and .45 ACP, which comes out to $700 for the rifle ammo alone), and the phenobarbital they took to calm their nerves. When previously arrested, they also had another handgun, radio scanners, smoke bombs, IEDs, and three different California legal license plates.
After being caught with all of this in their car (they were pulled over for speeding and one of them admitted to having a concealed weapon), they served 100 days in jail and had most of their property returned to them.
On the subject of spree killers and automatic weapons, I can assure you from personal experience that "spraying and praying" does not make the gun a magic bullet hose. The projectiles you're firing are often no greater than 9 millimeters in diameter. Accuracy is far more difficult than most people think, and even at 20 feet many shooters will have difficulty putting more than 4 or 5 rounds on target (which is not always a fatal injury;
this video should give you a better idea of the lethality of firearms).
Always fear the skilled shooter with a single shot .22 more than the amateur with an illegally converted rifle. And the former is hardly well regulated.