So he told you he wanted you back, so you went back.
Then he treated you terribly and abusively so you (rightly) left.
Now he's furious that you are looking out for yourself? He gets drunk and has a firearm.
Does all of this sound completely wrong to you? It should don't you think?
Block him, stay away, run far far away from him and never look back. If he ever threatens you, please call the police immediately.
You can become addicted to this kind of on again off again, little crumbs of affection type of relationship, but it's very bad for you (and obviously doesn't work for him either!). You deserve to be treated with kindness and respect, you will never receive that from him. Let him go and find his own way out of this. His drinking or using other drugs is his issue, not yours. Unless you tied him down and drugged him, it's not your "fault".
You didn't Cause it, can't Control it and can't Cure it (the 3 c;s).