kdvollmer

If this is going to be a single firearm, defensive, and have to last a long time, possible you can't get another one. Then get a mid size caliber rifle in a pump action. Semi auto, though reliable by todays standard will most likely wear out sooner than anything else. More moving parts, gas tubes...etc that can go wrong. A pump, well lets face it, are exceptionally reliable, and going to be easier to fix if you break it. Caliber is really going to depend on What is available out there, might even go so far as to say something in a .44 mag, though that might be a little over kill for squirrels, you can take rabbits with it with out too much damage, as long as it isn't a hollow point round. That is of course as long as that caliber is out ther ein a pump. Might not be. Other wise, .223 is good, you can kill most anything up to deer with it, good for defense, readily available, and if you need to reload, not to pricy either. Though .308 would most likely be my overall first choice. That brings even larger game into the picture. I have killed rabbits with one at 20 ft ( just stumbled on them ) and shot them with 165 grain match ammo, and really didn't destroy any meat. Also a good caliber since it is a military round for Nato, it is widely available.
For something like this, It would have to come with iron sights from the factory, and be drilled and tapped for a scope as well. Variable power in this case is a must. Say a 3-12 power. Low enough for close shooting and in the woods, and enough higher power for longer distance if needed.