I second the Remington 870 in 20 gauge for home protection.
Less blast, less recoil, still absolutely devastating at living room distances and unfailingly reliable.
I keep my 870 Youth Model 20 gauge by my bed, loaded with four rounds of No. 3 buckshot. On the stock is attached a stretchy-fabric shell carrier with an extra five rounds of buckshot.
Don't discount the 20-gauge, No. 3 buckshot load.
No. 3 buckshot is .25-caliber in diameter (in fact, it makes a very cheap substitute for reloading .25 Auto practice rounds. But that's anothe story).
The 3" case holds 24 pellets at 1,150 fps muzzle velocity, the 2-3/4" case holds 20 pellets at 1,200 fps.
These are not loads to be sneezed at.
The slug load is 3/4 ounce (328 grs.) at about 1,600 fps. This puts the 20 gauge in the realm of standard factory .45-70 loads -- hardly a Nerf load!
Have you checked the Remington website? I recently noted a 20 gauge 870 with extended magazine holding 7 rounds (if I recall correctly), black plastic stock and 20 inch barrel.
This would be the perfect home protection shotgun, in my estimation.
Don't get me wrong, I like the 12 gauge. But it seems overkill for the short ranges involved, and the extra recoil and blast can be a hindrance, in both actual use and especially in training.
I originally bought my 870 20 gauge Youth Model 15 years ago to hunt grouse in the thick woods of northern Idaho. I've put perhaps 1,000 rounds through it and never had a failure to feed or function.
I originally purchased a Winchester 20 gauge but didn't like it. Too heavy. It seems that Winchester builds their 20 gauge on the 12 gauge frame. Remington scales their 12 gauge down to fit the 20 gauge cartridge, making it much smaller and lighter.
My 870 Youth Model is about as long as an M1 carbine or Ruger Mini-14 and weighs just a tad more. I put a nylon sling on it, so I could have both hands free for climbing up steep banks or over obstacles. A quick-detatchable sling is a good addition, but may be a hindrance for a house gun.
No problem. Just add the sling for hunting and remove it for home defense.
The 870 is a magnificent shotgun. You have to decide the gauge you want, but I'm offering the above opinion so you don't readily discount the 20 gauge.
Either gauge will work, for my own uses I prefer the 20 gauge.
For greater versatility in ammo, choose the 12 gauge. You can get all kinds of esoteric (and often downright silly) loads for the 12 gauge that are not made for the 20 gauge, including incendiary and whistling rounds.
Whether you'll need these is up to you. But be warned that such rounds are pricey, often bringing $20 to $30 at gun shows for a packet of 5! Ouch!
The cost of either 12 or 20 gauge cartridges, with standard field loads, is about the same. Sometimes you'll find 20 gauge slightly cheaper because they may not sell as briskly as the seller wants, so he reduces them to make shelf space.
I've picked up more than few boxes of 2-3/4 inch No. 6 shot field loads at reduced prices for this reason.
For rabbits, grouse and pheasant, No. 6 shot is great. As a home defense round it can be devastating too.
Take a look at the Remington website. See what's offered. One thing: you can't go wrong with an 870!