870's and Mossbergs are almost the AR's of the shotgun world. Infinitely customizable and oodles of accessories for each platform.
An 870 with a 20" rifled barrel and cantilever scope rail for deer/bear -- swap out with a 26" Versachoke for duck/trap/skeet/pheasant/grouse -- swap out with 18.5" police barrel for "social work". I put a Blackhawk Knoxx Stock on mine...gotta love it! http://www.blackhawk.com/product/SpecOp ... 08,165.htm
Extra 870 and Mossberg barrels can be found at nearly any pawn/used gun store. Metro Pawn in Richfield has a bucket of them nearly anytime.