Best caliber tactical carbine in the AR 15... or other? + Barrel length?

I have decided to spring for a tactical carbine, I am looking at a flat top AR 15, I was curious I know you can buy them chambered for .223, 30.06 or the .308, I intend to use this gun as a SHTF rifle, so I won't use it for hunting or anything else just range use and in case SHTF use. I was also curious about... show more I have decided to spring for a tactical carbine, I am looking at a flat top AR 15, I was curious I know you can buy them chambered for .223, 30.06 or the .308, I intend to use this gun as a SHTF rifle, so I won't use it for hunting or anything else just range use and in case SHTF use. I was also curious about barrel length or the best combination of the caliber vs barrel length. I know the 16inch is better for close quarter but gives up range, so I am torn.
Update: I am open to looking at other weapons, but I like the fact there are so many accessories to the AR 15. I have looked at the M1, M1 Carbine, some of the foreign brands are just to expensive I am looking to stay in the 800-1200 dollar range.
Update 2: one more thing, not looking to break the bank on ammo thats why I am looking at the .223.
Update 3: Arthur - looked at the mini I like them, they go for around 600 here, I know you can buy stocks and accessories that make them comparable to a AR. I know that the local walmart in Montana sell ARs for around 600 as well though
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