I can personally vouch for my Remington 870 Express magnum, which I have used in most reasonable (and some unreasonable) weather conditions and never had a single problem.
I find my 870 simple to strip, While to completely strip the gun you do need some sort tools (Not special though just a Screw driver and a hammer), However you rarely, if ever will need to strip this beast in the field where tools are unavaliable. There are also an almost endless list of after market parts that are compatible with this bad boy.
If an 870 sounds a bit weak too you, Try looking at Mossberg 500's. I've never used one, But I've heard that their Brutes.
Good Luck choosing your WMD.
(If you do look at an 870, Make sure it's a Supermagnum, becuase they cycle 3 1/2'' rounds.)