There is no purpose for owning more than a handgun other than hunting.
1. Long range self defense
2. Sport/Range Shooting
Your point about the stuff in public is pretty fair. Those guys wearing the weapons on street corners protesting the government, or open carrying in a starbucks are nothing more than complete flamboyant richards and mall ninjas.
However in many places you can just walk around with a gun. You have to check with law enforcement and get an approved conceal carry license. To get one you have to have a legitimate reason for having one.
Legimate reasons might be:
1. You are a security contractor/body guard
2. Multiple attempts on your life have been made by gangs
3. You are politicians who is too poor to afford body guards
Depending on the state like California, you don't need a conceal permit if are a business owner and you want to defend your home and business.
on full-auto weapons:
Any weapon that is fully automatic, fires explosive projectiles, has a large barrel, shoots flames, etc is considered a destructive device and labeled as a class 3 weapon. These weapons are illegal to own. However you can posses these weapons by applying for a permits to own one these. You have to go through a very deep background check and you have to pay a 200 dollar tax stamp.
On home defense: If you are smart, you don't have to kill the intruder. You can just point the gun at them and tell the leave. If just tell them to leave while being under armed that won't send as strong of a message. You can only kill someone in your home if you feel like your life is in danger and killing the person was the only option left.