IDK, time, I think Adam was a very bright, resourceful, immature adult who would have found a way to get his hands on a gun. As another poster said, he did try to buy one himself from an arms store but just didn't want to wait for the approval of his registration.
Maybe he would have found ways to break the gun safe. He did de-magnetize his hard drive and demolish his computers. Or maybe he would have bought a gun off Craigslist or Ebay or wherever.
I think where there's a will, there is a way. Adam, despite his social issues, still managed to learn to drive and shoot guns, and apparently was quite smart in school -- until his parents' official divorce and then his grades seemed to veer off by the wayside. So he wasn't so disabled he couldn't function on his own.
He didn't try to buy a gun. That story was debunked. It was a case of mistaken identity-somebody else tried to buy a gun. Not AL. Why would AL need to buy a gun? Considering his mother kept all kind of guns in their house.