I have a 686 ultralight 12 gauge that tips the scale at 5 lbs 7 oz.  It is a dream to carry and is the fastest-mounting gun I've ever handled.  However, it kicks like a mule.  Not a problem for hunting, but I like to practice on clays in the off-season, and anything more than a box is just not fun anymore.  Other thing I've found is that I don't shoot well on crossing birds with it - I think it's too light to swing well.  I changed to a 687 silver pigeon II for my pheasant gun (about a pound heavier) and I shoot much better with it for pheasant.  I kept the ultralight for woodcock and grouse though, just for the speed of mounting it for snapping off a quick shot in the alders.