forget the 870 and get a mossberg 500.
mossberg got it right in 1961 The 500 is everything a pump should be, never worry about it jamming up..its freaking impossable. You can even let one loose from the mag -- it just hits the bolt until you cycle it out.
I realy dig the 500's action -- I think the 870's ramp is out dated, they can jam-- but 870's still kick ass -- theres lots of aftermarket stuff, and its easy to do --the are tank looking next to a 500...with its aluminum recievier-- it doesnt need a steel one, its badass like that and its lighter.
some other favs --the best features of the mossberg -- is the 2 claw shell exctrators...and the ejector can be replaceed with a screw...I got this remington used one time and I didnt notice that the ejector was broken...and you cant replace that with a screw, on the remmy. I returned that 870 and got my money back.
mossberg 2 extracors -- rem. 1 extractor. duh?
get cylinder bore 18.5-20 for shooting buckshot and slugs at close ranges. Get a modified or full choke (longer brls) for longer ranges.. although--a buffered 12 guage magnum shell will shoot tighter at 40 yds from a cylinder bore than a full choke 30 in brl..with no buffering. That buffering is good for long range shots...dont use then in th house...Buy Sellior and belloit 12 shot 2 an3/4 in.00 for the house and spread.
you can also bend the end of your brl if you want even more spread, sideways or then lay it up gangstersyle to get up and down..full body spread.