Who shot Gun B?

As the Sterling High School shooting trial moves along, more students are interviewed. Drew Girard was chosen to be asked questions and Jordan McAfee makes Drew look like a bully. The types of questions he asks like "Did you ever call him names?" "Shove Peter into a locker?" "What about the time you pantsed him?" put Drew on the spot but he answered all of them honestly. From what I've learned and already guessed, Josie's "crowd" (the one that Matt, Courtney, Maddie, Drew, and others hung out in) were very mean to Peter. They bullied him pretty much everyday.

Next, Derek (one of Peter's only friends) came up to the witnesses stand. He was asked the normal questions and everyone heard that along with Peter, he was picked on quite often as well. Jordan started asking him if Peter had ever talked about the shooting and he said no but the two played his game, "Hide-N-Shriek!" Then, Derek said something at the end of his talk. A female lawyer asked, "You never put together a plan...killing people who had hurt you the most, did you, Derek?" And Derek said, "No, but sometimes I wish I had."

Is that saying everyone who was picked on or bullied wishes they would of done what Peter did? Was Peter sticking up for those kids?

At a jury meeting about a week later, the judge asks Patrick about the guns Peter used. He said Peter on used two guns, Gun A and Gun B. Patrick and the crime lab have proof that Peter used Gun A, but he still hasn't discovered who used Gun B. That seems to be the missing piece of this puzzle.

At this same meeting, a physiologist has done some research on what may have gone wrong and if Peter had a brain disorder. He concludes to the judge that Peter was just going through the motions and actions of everything he did that day like shooting people but that he never was capable of thinking about what he was doing.