When I younger weight never seemed to be an issue even though we would cover alot of ground thru thick cover & fields. A bit later on I started hunting over a dog using an O/U I found myself with my gun open resting in the crook of my arm 80% of the time in the field and again weight wasn't a concern to much. For almost 17 years I was using my 12 ga.sporting clays gun for almost everything even though it weighed in at 7lbs 10oz. with 28" barrels but did use a 20 ga that weighed a full lb. less on ocassion. I found for me 1 oz of 7 1/2's for the first barrel and 6 or 5's in the second was always plenty for upland birds.
All that being saidbeing older I'mgoing to treat myself after the holidays to two new O/U's , one 12 & 20 (thinking of a 20 & 28 two barrel set) that will tip the scale at just 6lbs. Just remember when you go light your skills have to be up to par as you'll get no help from the gun for your follow thru and if you don't have confidence shooting loads that aren't going to knock your molars out light might not be your cup o' tea.