i would have to say a .22LR in semi auto with at least 10 rounds in the mag. The reason would be that you can carry so many rounds in your front pocket. And being semi auto you could get off a huge number of shots to protect yourself and be pretty accurate due to low recoil. Plus you have some range. you could take a squirrel at 50 yards no problem and still carry 500 rounds in your pack.

A shotgun would be great, but really how are you going to carry more than 50 rounds plus anything else you think you may need.

i think a Remy 597 VTR (ar style) with a 30 round clip would suffice.