Tut, the reason a 16 gauge Fox weighs about the same as a 20 gauge is that the breeches are lighter on the 16. What is preventing you from striking the breeches and breech balls on a 20 gauge until they are the same thickness as the 16? There has to be some weight there. It would also make the gun a bit sleeker in that area. I was probably being a bit optimistic when I said "under 5" but that was what I meant. How about under 5 1/2? With stuck barrels, reduced breeches, ribless design, and skeletonized stock, 5 1/2 pounds in a 20 should be no problem.
I think the 5.5 lbs is certainly doable on a 20 gauge SW I have with 26" barrels. It already comes in at 5 lbs 15 oz. I think with action work and if (which I won't) I converted it to english grip. What I'm trying to figure out is how to get my 16 down to 6 lbs with 28" barrels that weight 51 oz. That's the challenge of the day. I'm supposed to be headed to PGS next week to meet with Dan, but I might have to move it back one week. Will know more in the next few days.
PS. I've got a 26" 16 Gauge A grade Fox (extractor gun) that letters at 5 lbs 10 oz and this is with the half round stock. Ansley built them light back then for sure.