I would buy an arm, but I would rather prepare with gold, jewelry, and especially canned water and food, and have enough in your accounts to pay for water, electricity and mortgage in the months/years to be crucial.
So once you've prepared your stash of gold, jewelry, food etc. how are you going to keep people from taking said gold, jewelry, food etc?
A gun is legitimate self-defense, but there is no need to build a fortified house with machine guns etc.
I'm assuming you don't live in the US or you are very young? (I don't intend insult by either assumption) but your vision of someone 'fortifying' their home with 'machine guns' is a bit, well... silly!
If they want you, the Homeland Security via the Army will get you. [ANOTHER INSTANCE OF SILLINESS] There is no purpose in arming
There is a purpose - it's called self defense and every man has a God given right to it. Someone who is unwilling to defend his own home, state, family etc..has a name in America: a coward.
You should have enough in your accounts, make emergency rations, have an evacuation plan if you live in a large city, and you merely need ("need" is relative in fact) a small gun to defend your home and family in case of civil unrest or uprising.
I'm not sure where you got your idea of only needing a small gun (and in the case of "civil unrest" or "uprising" especially)but it's really naive! I think you've been spoon fed anti-gun propaganda and consumed the contents to excess!
I hope you don't have a family to defend! I would hate to be a kid in a house with murderous assailants kicking in the door and thinking how daddy didn't think us kids were worth it to defend us with something more than a red rider bb gun!