Emergencies. If I need to actually use it, this country will be in a world of hurt already.
For low cost and reliability, I’d look closely at the Ruger Mini-14 in .223 (5.56)
Well, don’t forget to practice.
don’t waste money on a long barrel, and get the flat top receiver. and don’t waste your time building your own, there’s not enough cost benefit unless you’re really strapped for money, and if that’s the case, get a mini 14 or an AK variant. they aren’t as accurate as an ar, but are cheaper and more reliable.