Should I leave him for being this way?

You should leave, and do it when he's not around and not likely to be home for a while. Seriously, I think you're too close to the situation to realise how terrifying it is. An unstable guy with extreme anger issues, a gun, who's off his medication, and blames you for everything that's wrong. Pregnancy is the number one trigger for domestic violence in relationships. Even if it's wanted and planned, for some reason, if it is going to start, pregnancy is what usually does it. And it escalates. You may feel you can deal with it just now, but it wont stay like this, it will get worse. Definitely move back in with your mom, (without him knowing, leave a letter if you like, but dont give an unstable, angry guy with a gun the chance to think about stopping you) and inform the police that he has been angry and threatening, has a gun, and is off his medication. It sounds a seriously scary situation to be in, and I think it's really important for you and the baby that you get out of it and take precautions to keep yourself safe.