Wow, this is good stuff.
As regards the original question about the pendulum, I'm going to say 30, maybe 40 years.
As shotgunners entered the 1900s the decided preference was for 30" barrels. Somewhere abound 1925 or 1930, give or take, shorter barrels gained traction and stayed on a pedestal for the next 50 years.
What strikes me in particular is that during that 50 years a lot of the literature was authored by outstanding field and competition shooters, literate too. And as a group they were all gravitating toward shorter barrels. It is not as though only short barrels were available or only open chokes could be had with short barrels, because that simply was not the case. Short barrels were preferred. Speed was a big issue.
Though some of the long barrel advocates here make good points, I think it is presumptive of us today to think we have finally invented the wheel by adding a few inches to the tubes. To do so would be to wholly dismiss the considerable experience and expertise of the two generations of shooters who preceded us. Personally, I side with the guys who are saying it is a preference, or it depends on the particular gun.
To make a point, I need to insert a little story here. A friend owns an outfit that caters to bullseye shooters - really serious, high-end stuff. I was shooting high-power then and my loads and equipment had pretty much gravitated to what everyone else was using. I related to my friend how I had tired this powder and that bullet along the way before I fell into the trench with everyone else on the firing line. He said something like, "Yeah, if all the shooters would just use what's known, practice with it, and skip all the experimenting, I'd go out of business tomorrow." The point is, experimenting is elemental. We all like to think we can build the perfect mousetrap or gain a secret edge. The thing about rifles is you can throw them on a bench and prove the point. The thing about shotguns is you can tweak them six ways to Sunday and confidently dare anyone to prove you wrong.
All that said, my prediction (eyes closed, hand to forehead) is that we will build a generation or two of long-barreled guns before our great grandchildren look for the better mousetrap, discard all our carefully conceived arguments, and start snatching up 26" guns like there's no tomorrow.
