or the poor boy alternative-----sks............
The good ole' sks will be cheaper and make a fine SHTF weapon. Just get stripper clips for a faster reload and don't worry about high dollar magazines or all the other tacticool stuff. The money saved going this route will enable you to get that 870 along with a good supply of ammo.
Each weapon system (AK, SKS, or the AR/M16)has its strong points and its champions. Each weapon system has been killin' their perspective enemy for a lot of years.
I would also look at it a different way. A SHTF gun is a good thing to have, but you might as well get in some practice, enjoyment, an usage out of the gun before it hits the fan. So looking at the AR series besides SHTF you get:
a varmint rifle/competition rifle for shooting tight groups at distance(varies)
something to customize to your exact tastes
AK or SKS series besides SHTF you get:
a superior caliber legal for deer/hog hunting (7.62x39)
cheaper shooting so more practice
My daughter has already gone the SKS route. She specifically went this route as the AK's, mini 14's, and AR's were too high. She got herself a little military canvas bag and filled it full of loaded stripper clips. And is still facinated by the "xtras" such as the cleaning rod and kit that come with it. And for a SHTF rifle, those little xtras start to have merit. More affordable ammo a big bonus as is evident by the trips to the range.
I've created a monster......................
I have also been looking into another alternative---the .357 lever action rifle. Rather weak compared to the true rifle rounds, but for my current/forseeable needs, still a viable alternative. I'm in a position where "head to the hills" is not an option. For better or worse, i will be staying "urban", (if you can call my town that). And dual handgun/carbine ammo usage a plus. But thats for another discussion...........