Because it is a cause, but not the only cause, we have some degree of screening in place for this.
No we do not.
There is no screening in place to prevent an undiagnosed mentally ill person from getting a gun.
The only way someone like that could be prevented from getting a gun is if they were already diagnosed.
My point is that undiagnosed people don't obviously have a diagnosis, so any "screening" won't pick that up, will it?