Even with a bolt gun, I would imagine it be a good idea to have a 7.62/.308 AR.

So, with all these pictures of people with 20 or more AR 15's... what's the difference?

I know basics such as it would be ideal to have:

1 (maybe two: 1 work, 1 match) - .223/5.56
1 - .308

I know there are 6.8 out there too, but will they do something that a .308 can't do?

So in these pics are they all just teh same calibers or different or what?
I'm less interested in the reasonings such as - 18 inch barrel has more velocity than a 14 inch, etc.

Because if that is the reasoning, can't you just switch out uppers? Why a whole other weapon?

Yes, I'm an AR-15 noob.

Just curious.

I know, I know: THE MORE, THE MERRIER!!!

But if worse than worse ever comes, I do NOT plan on outfitting my whole community and I can only carry so much...