I feel the % increase in British guns is less because Brit guns were usually upland guns and lighter than their American cousins. In our country, 12's (and 10's) were all around guns for deer and waterfowl while sub-gauges were for upland game. In Britain, 12's were mainly upland guns and 6 pounders were not uncommon. A 20 in a classic American shotgun is both rare and neatly balanced and light. They are worth more and sell quickly.