I agree with runreilly.
Sometimes people can forget that after Columbine, it basically just became a body count contest. Sure, Eric and Dylan wanted to kill hundreds of people if they could manage it, but guns were only meant to be supplementary. This is why I don't think they were intent on making the most of their time as if everyone was a winner in the murder lottery. Eric and Dylan were counting on their bombs to do most of the work for them, while Cho and Lanza strictly used firearms to achieve kills.
I think they were ill prepared for up-close kills and no amount of planning would negate that. While they did manage a few, the psychological intensity of personal killing threw them out of their element for a while, hence the wandering post-library. I mean, these are kids we're talking about. E/D would resent the designation, but they were kissless virgins who still lived with their parents at that point. They talked a lot of game, but Eric still broke his nose and many were merely taunted and not shot. There was too much screaming and crying and alarms blaring to think properly and they're expecting a fiery confrontation with law enforcement any minute in the midst of this. Their actions when all's said and done make sense considering thin, lanky computer nerds with a lot of anger and resentment but little confidence and bravado.