Hello. I believe I can be of help to you with this issue.
I would talk to the police about your husband. They may act alone or consult the physicians at the local psychiatric inpatient unit, about trying to get him in to treatment or rehab. You can and should make this report because you have what is called a 'reasonable suspicion' that he may be a danger to himself or others. You don't need hard evidence or proof that he intends to hurt or kill himself, but the laws state that you need to act based on an honest, reasonable suspicion or concern. The police may do nothing, but they may at least pay him a visit to check up on him.
I can tell you that you have done everything 100% correctly. You cannot allow him to manipulate you emotionally through suicide threats. In the event of suicide 'talk' or threat, you continue to do what you know is best---just tell him that if you feel he needs to be made safe, you will help him make an appointment immediately to talk to someone (a psychologist who has expertise in mood disorders), or you will call an ambulance for him and he can go to the hospital for observation. Tell him that in no way do you want to see him lose his life, but you cannot give him the help he needs---which is help and support for his serious, clinical depression.
You may go so far as contact the friend he is living with, alerting them to the need to try to coax him in to see a clinical or counseling psychologist immediately, or a therapist at the behavioral health unit of the local hospital---for an evaluation.
He should not see his kids while drinking or drunk, of course. Again, you were very wise in setting the limit that he cannot come back and be 'lovers' and help raise the kids right now. You can emphasize that you truly hope he will never hurt or kill himself, because his kids do need a dad---but they need to see their dad get well and recover from his depression and other problems.
And, you need to unload the gun and get it out of the house altogether---have a friend keep it or put it in a safe deposit box at a bank. So here too, you did the ideal thing---refuse to give him the gun and offer him some money for it instead.
What do you think?