I would like to think the original owners of these guns knew what ammunition they were made for and acted accordingly. But when they passed them on, most likely without the hang-tag they came with telling what shell to use they started digesting any old ammunition the owner could stuff in them. Our North American ammunition manufacturers offered 2 1/2-inch 20-gauge shells up to WW-II, but after WW-II only 2 3/4-inch 20-gauge shells were offered. I just measured the length of a fist full of nominally 2 3/4-inch loaded 20-gauge shells with the post WW-II pie crimp and got anywhere from 2.305" to 2.335" all would easily fit into that 2.375" chamber. Those shells could be anywhere from 2 1/4-dram equiv. 7/8-ounce loads to the maximum 1 1/8-ounce 2 3/4-inch Magnums. I suspect that the recoil forces of a steady diet of such shells from the late 1940s into the 1990s when we started worrying about such things did in the heads of a lot of vintage 20-gauge double's stocks.

Roll pin
 

BTW, the roll pin or hinge pin on that gun looks to have been reversed, since the slot is normally on the left side of the gun.

Agree 100% with the impact on the wood theorem, but so far as chamber effects it brings to mind the old quip about a thermos keeping things hot or cold - "How do it know?"

I was attending a talk given by former player and coach Mike Ditka. He told a story that during a team meeting when he was with the Bears, the talk got around to a discusion of what the greatest invention of all time would have been. When William "The Refrigerator" Perry's turn came, he said he thought it was the thermos. Fridge was asked why he thought so, and his reply was the same thing; "It keeps hot things hot and cold things cold, How does it know to do that?". True story as told from "Iron MIke".