I have extensive experience in the field of mental health. I work in the field. My daughter has extensive psychiatric problems. I associate with doctors in the field.
Forcing the system to recognize a nut is tilting at windmills. It won't happen. Reporting a nut with a gun may work through legal channels, but probably not, as a person is legall free to both be crazy or a criminal, and own a gun, until he breaks a law and is convicted.
If the situation is genuinely serious, contact the chief of police or other high ranking official directly, explaining briefly and in small words, your greatest concerns. If he has broken laws, file complaints. If he breaks laws, file charges. Get legal attention to his actual criminal behaviors.
Do not report him as contemplating suicide. Why? he will be held for several days, and in most cases, he will be released with a clean bill of health, no matter what is wrong with him. He isn't suicidal, and that is why he is there. He can choose not to seek treatment for any other problems. Any information gathered is privileged, and will not be shared with police. On top of that, this individual will find himself responsible for a bill of many thousands of dollars for the confinement. No, the cops don't pay, and the hospital doesn't do it for free. Do you really want a nut to find out that a coworker turned him in as suicidal, had him locked up for days, caused him untold hardship personally, and then, got him saddled with a $10,000 debt?
Observe the AZ case. He was a known nut. he had been in the system. he had been involved in both legal and psychaitric interventions. yet he still committed a mass murder, in spite of the fact that numerous people knew he was crazy, and had struggled to stop him. This is reality. this is how it works. Even with legal and psychiatric fields knowing there is a problem, it can and will still happen. Usually, it's not on such a grand scale.