gonna go out on a limb -
Keltec RFB... with a Millet DMS1 scope on it..
yeah... you heard right - an RFB.
here is why:
Lightweight, compact firepower.
I've heard good things about it's ergonomics, (especially for lefties - which my Wife is) it's compact size makes it easy to handle (my wife really can't even handle my Mini 30 correctly due to length) I've heard it has a pretty nice trigger pull, and they are pretty rugged and easy to field strip and clean
at 26" long you can stuff it quite a few places out of sight should it be appropriate to avoid attention. but it still has an 18" barrel
It'll fit completely inside my 3 day Pack with no problems at all
(try doing that with a Full sized M1A at 44" long or even a scout squad at 40" )
it shoots 7.62x51 / .308 (which hopefully should be one of the more common rounds that can be scavenged later) a round that has proven to be adequate for any four or two legged target you may find necessary to shoot. - I did think of sticking to 5.56/.223 but then thought about the "shot placement" factor of hunting for large/medium game with a small bore rifle - if I need to RELY on hunting for some meat - i'd rather shoot a cartridge that will do the job even on a marginal hit - instead of having to waste precious energy and time tracking an animal that didn't go down right away because you didn't bring enough gun, and you are cold, hungry and exhausted and didn't put it though the 10 ring.
it uses FAL magazines - which while not as cheap or prolific as HKG3/Cetme mags but they are about as common as M1a mags or AR10 mags. hopefully they can be scavenged relatively easy too.
parts availability would be an issue - but somehow despite the movies... I really don't think you are going to get into any Balls out firefights on a regular basis, and if properly cared for guns are pretty simple to keep running and can last a lifetime. I really think in a survival situation, your gun is going to see less use than it does now (monthly range trips where you rattle off 2-300 ounds like it's nothing will be a thing of the past, no?)
I foresee you'll be using couple of well aimed shots per month to harvest some meat - or maybe a mag load once a great while to convince some scavenger that what you have in your pack is best kept in your backpack and not his.
Ua Mau ke Ea o ka ʻĀina i ka Pono