EDIT: I see my post is being downvoted. I know that it may come across as insensitive to immediately leap to the gun control debate, but frankly I'm more angry than upset by this news. How many times does it have to happen? We have a good 48 hours of emotional outpouring and then everyone forgets it ever happened.
How many times are we going to find out someone had a history of mental issues and nothing was done? How many people are reluctant to take proper medication because they view the effects of the medication worse than what it trying to treat. We have a lot of people coming back with PTSD, we need to get better at this.
I remember OK and that was farm products (which some companies still sell at automated machines). IEDs, pipe bombs, propane canisters, Molotov cocktails are all capable of killing a lot people and are easier to build or acquire than guns.
But mental illness itself has no check point. How can we ensure that people do not have untreated Schizophrenia? Mandatory checks for everyone once a year? I'm really not sure how it could be done.
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.