In some ways, ammunition IS a major part of the problem. There are enough guns of various types out there that we could search forever and not 'take away' a significant percent..... but unless they can get new ammo to replace what they use, (and some of these guys cannot bear to give up 'practicing'), it could lead to higher prices and reduced supply.
(and no... most of these guys could NOT 'load their own' for most types)

There is no single, simple way to reduce the horrible statistics.... not even with a guard at every school. **IF** every school had gates & guards, crazy fools who just want to kill will find other targets and locations... buses, playgrounds, more malls, theaters, stadiums...etc.
It would be a huge 'live' version of Whack-a-mole!... and there will never be a shortage of crazy fools who can beg, borrow or steal weapons.