Macro: If you’re going to Discount Gun, ask the salesperson about either the Remington 870 or the Mossberg 590. You want to buy a 3″ receiver (called the Police model by Remington), so you can shoot both 2.75″ and 3″ shot and slug. Under no circumstances buy anything with an aluminum receiver. I prefer Remington to Mossberg, but largely because I think the Mossberg’s action is rough and harder to cycle than the Remington.
Explain that you’re looking for a shotgun for personal protection and, if you’re thinking home defense, you want to be shooting buckshot and not slug (you run the risk of over penetration with slug loads, which are essentially very big bullets). Federal makes an excellent off-the-shelf buckshot load in both double-ought (00) and triple-ought (000). Have the salesperson explain the differences between 00 and 000, and the differences between standard loads and magnum loads. They’re very knowledgeable at Discount and they’ll cheerfully give you a 15 minute primer on shotguns and ammunition.
Once you buy the shotgun, make sure you spend sufficient time on the range to get used to it. a 12ga is a formidable weapon and you don’t want to start out shooting 3″ 00 buckshot for instance. Buy some cheapo Peters birdshot in 2.75″ and run a couple boxes through. Work your way up through 2.75″ standard loads until you’re comfortable with 3″ magnum 00 buck. Trust me on this, that load will stop anything dead in its tracks and do it with authority.
If you do wind up with an M-60, make sure you use either the bipod or tripod and don’t fire it from the hip. It looks cool, but you won’t hit shit.