People often complain about steel cased ammo. They say they don't work in AR's, but if its a proper AR or any rifle worth the money it will shoot steel cased no problem. True, it is not as high quality as brass, but its great for target practice and you can get a good round out of steel cased if you get a soft point or hollow point.
But I think if you were ever in said situation...I'd be more worried about trying to clean the AR in less than ideal situations with cleaning supplies running low and probably hard to find than what ammo I had (as long as I had some). Stock up on what you can afford and always make sure you have a back up of some sort. Whether it be a shotgun, pistol, knife, or another rifle.
Ever watched Nutnfancy videos on youtube? He runs a lot of wolf. Sure he's long winded, but his intention is good.
I wouldn't suggest silver bear though except for plinking. The zinc will corrode if it gets wet.