Get thee to a Wal-Mart and buy a Remington 870 pump shotgun in 12ga. Pick up a cleaning kit, Rem-oil, Tuf-cloth, and silicone treated storage sleeve. Buy a box of #7 bird shot and a box (5 rnds) of copper sabot slugs while you're there.

Head down to the gun shop and pick up a box of #2 bird shot, a box of #2 buckshot, rifled and full choke tube to fit and the pouch to carry to carry the chokes and wrench that come with the gun. While you're there get a magazine extension and have them fit swivels and a sling.

With that kit you are ready for anything from crows to bear and ranges from point-blank out to 100 meters with slugs for medium to large game.

At some later day you can get more ammunition and fittings. Later still, fancier iron. But even after you have moved on your simple pump shotgun and a pocket full of shells will likely still be the go-to setup for home security, backup, and investigating what is thrashing around outside your camp site.