I have no problem with hunting rifles and the like. If the killer walked into the school with a hunting rifle, he'd have gotten off 2 shots and then would have had to reload. While he did so, someone could have done something to stop him.

With the gun he had, he was able to fire 100 rounds before reloading. So he could go through the entire school (theoretically) without reloading once. What's the use of a gun like that except to kill a lot of people very fast?