tarkio,
Welcome to THR-
The most important part of any defensive shotgun is 'the nut that holds the butt." In other words, the shooter. Your skillset is more important than your hardware.
As to the shotgun, a barrel from 18- 21" is about right. For a genuine 'house gun' a Cylinder bore (no choke) will work OK at close range. Most Remington factory short barrels these days have Improved Cylinder fixed chokes, older ones will still be CYL. Choke markings on 870s are stamped into the barrel just in front of the receiver on the left side of the chamber area (left looking from the butt of the gun). Note that a barrel may have been shortened and had its factory choke removed in the process, so the marking might not be accurate. There are a good many 'slug' barrels out there, most are 20" with rifle type sights. Be sure you stick to a smoothbore barrel though, some of those with sights are rifled. Rifling's good for sabot slugs, but bad for buckshot patterns.
There are LOTS of parts (including extra barrels) for 870s around, if you need a replacement stock because yours is unserviceable it shouldn't be too hard to find. eBay lists a couple pages of them, larger gun shops and gunsmiths in your area may have some in stock, too. Just make sure the gun fits you properly and has a good recoil pad.
Start off using the gun without a magazine extension- the practice in loading and shooting will be good for you. To quote shotgun sensei Louis Awerbuck, "No magazine is ever big enough." Learn to feed that puppy as it barks (shoot one, load one) and you'll be better off than trying to find a way to cram in just one more round. You can always try a magazine extension later, after you learn not to depend on one.
Look for a way to mount a good white light source on the gun. That's about the only extra that's on the 'short list'. A weapon light is no substitute to having a handheld flashlight too, but IMHO a defensive shotgun should have everything on board you will need if you have to grab it at 0dark30.
IMHO that includes a reload- either in a butt cuff, Sidesaddle or Speedfeed stock. Each has its own adherents, some like none of them- but I want a reload on the gun when I pick it up.
The gun sitting at my left elbow as I type this has a 13" LOP stock, an 18" Modified choke barrel (factory fixed choke) and rifle type sights with Tritium inserts (also from the factory). It only took me a few years of serious looking to find one of these barrels, BTW- it's an 870P barrel and they only have 'em in stock if some PD or SO orders a bunch of 870s with this barrel and there are extras available.
It has a 6-round Sidesaddle, a Remington 2-shot magazine extension and clamp, with a clamp-on mount that wears a Streamlight M3 light. It's built on an Express receiver. There's five rounds of 00 buckshot in the magazine and 6 Brenneke KO slugs in the Sidesaddle.
That's not saying it's the be-all and end-all of defensive scatterguns. It's not saying it's what you need or should necessarily want. I live on 5 acres in the country and a shotgun is my go-to gun if I have a chance to reach for more than the S&W snubbie in my pocket. I might need to make shots 50- 100 yards away with this 870, and so I want more than across-the-room capability from it.
hth,
lpl/nc
