The mossberg the military uses is not your off the shelf mossy 500 for starters. It has been upgraded with some metal parts instead of plastic ones. However the action is pretty much the same. It also passed the mil spec tortue test that the military runs it's weapons through. Remington did not submit the 870 for testing. I am quite sure it would pass as well.

Functionally the guns are near identical, one is no better than the other as far as dependability goes. And take my word for it, a mossberg will fire when wet. It will also fire when covered in mud and ice, been there. If anything the mossberg would work better in these conditions because the tolerances are not quite as tight as an 870 (from what I understand). Much like the AK, it has loose tolerences so it can be made cheaper and function better in nasty conditions. It gives up accuracy for it though.

From what I have seen when a 500 or 870 fails to function it is usually operator error. They are pretty simple designs for the most part.

They are both being replaced by the Benelli M1014 semi automatic though.

Paul