I sold a 16, single trigger, two barrel 21 six years ago for just under 7k. The longer, tighter barrels were like new from lack of use. The shorter barrels were well worn from many years of heavy use before I bought it. I figured the Winchester, model 21 name added a third to the value to the buyer. I did not use it much because it was much heavier than a comparable Smith or Fox. If lead shot were legal I always though it would have been a heck of a duck gun for the blind. So 3500-4000 does sound about right but it always comes down to supply, condition and buyer/seller expectations of fair prices. Funny thing to me is when ever I had a double trigger 21 for sale it was always a minus in the sale but when trying to buy it is always claimed to be a plus.
