We haven't put the money into brain research to figure out if there is a test we could do. Checkpoints are the easy part. Look at all the places we test for drugs (employment, school sports). If we figured out the mythical dipstick that tells if your brain chemistry is broken, I would imagine it would come up in a lot places that drug testing does now and probably be part of peoples regular checkups. We just don't have a clue and no amount of dealing with the how will fix the problem, we need to get the courage to look at the why.
EDIT: That article you linked to discusses a 3d printed lower receiver. That part of the rifle does not have the barrel and chamber. The part of the ar with the barrel and chamber is called the upper.