Even .50BMG won't cost $5 each, nevermind any .308 at all. That's not the issue, though. My first rifle was a 5.56 AR, and I have multiple of those now, plus .22s and a lot of other things. .22LR will absolutely not provide you protection. It's best for practice and hunting small varmints like squirrels. You want to stand up to anyone, the 5.56 is the least you need. An SKS fires the 7.62x39, same as an AK47, and would be quite adequate to your needs. I have one of those as well; don't let anyone badmouth it. Sure, there are dogs, but they have served in the militaries of several countries with distinction.