Need advice in building an AR 15?

I have 3 acres of land. The only purpose of this weapon is for home defense. I am not talking about defense against one or two people such as robbers. I would use a pump shot gun for something like that. I am talking about if society collapses and I need to defend my home against gangs of people. What kind of... show more I have 3 acres of land. The only purpose of this weapon is for home defense. I am not talking about defense against one or two people such as robbers. I would use a pump shot gun for something like that. I am talking about if society collapses and I need to defend my home against gangs of people. What kind of specs should I look for in an AR 15 such as barrel length, material, rifling etc…Thanks in advance.
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