Gun may or may not help you if the bullet is coming out of an automatic assault rifle or what have you. If things really get at bad as I think they will get - and I hope they don't - you will first find governments banning weapons or carrying weapons without special permits. Police and military would be called to impose "peace" by any means necessary. People today are more violent than they were in the 30s and the human mentality would be to protest and rebel except today governments have weapons that will subdue large mobs by lethal or non lethal means. So your magnum might be useful if you have a robber who knows you have gold but I would imagine most won't know you have that. Just like you don't know who I am and where I live, likewise you don't know what your neighbor has or has not. Many will grow food and that might be your bigger concern to fight off those trying to steal your veggies ;) than them stealing your gold -unless of course you carry gold coins on top of your coat for the world to see. And I would guess that many people who do rebel or try to stand up against the government will end up in the detention centers that are already build and empty around US (may be other lands have such too).
One other thing about the Magnum theory, if I was going to have a gun I would definately have a gun with a silencer not one that would make enough noise for the world to hear and get your ass arrested for trying to save you gold coins ;)