As a guy who's put thousand's of different rounds downrange with more weapons than I can count, here's my two cents. Firstly, pick something you enjoy shooting. Simply put, if you don't enjoy shooting the weapon, you won't want to practice with it. Without practice, you'll likely have a failure to feed/fire, drop the magazine (thinking you're taking the safety off), forget to rack it, etc or shoot yourself or a loved one before you even hit your attacker. For example, I have a .380 pocket pistol I intended to carry but gave up on it after I realized I hated shooting the thing and therefore never practice with it. On the other hand, my 1911 roars like a lion and is AMAZING fun to shoot.
Secondly, regarding caliber, a good friend once told me, "If someone is shooting at you, you're going to run away". This, of course, is only applicable to humans. Humans, mind you, that aren't on mind altering drugs. Hence, if you're in bear country, get something to take down bears. If you live in the city, get whatever you want and just make sure you know how to use it.
Thirdly, for home defense, I can't argue with the Remington 870. I have one, and I love it. However, if I could only pick one weapon for personal defense, I'd forget about rifles and shotguns and go with a Taurus Raging Judge Magnum 513 with the 6" barrel. It shoots 3" .410 for home defense (you get five 000 buckshot pellets). At close range, that's like hitting someone 5 times with a .380 in one shot! You can also shoot .454 casull which should take down any bear, rhino, or crack-head you want. It also shoots .45 LC which is a slow moving but huge projectile. Great stopping power while not likely to over penetrate. The Raging Judge is HUGE AND HEAVY for a pistol but WAYYYY smaller/lighter than any shotgun or rifle. Easily transported and concealed in the glove box, under the seat of your car, in a backpack, etc. Also, being so heavy, the recoil shooting the .454 casull is still controllable and "comfortable". Also, the long 6" barrel should provide excellent accuracy and muzzle velocity. Finally, as with all revolvers, reliability should be top notch. And if you have a fail to fire, there's not slide to rack, just keep pulling the trigger and the bear won't eat too much of your leg.
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