Your guess is a good as anyone's Bill. Everyone likes to quote Bell's study of 12 GAUGE shells and NO ONE AFAIK as conducted a similar study of 16g or 20g, at least in the last 125 years
https://books.google.com/books?id=inQCAAAAYAAJ&pg=PA280 A trial of 1 1/8 oz. 3 Dr. Eq. 2 1/2” shells in 2” chambers with “an exceedingly long and gradual cone” showed very little increase in pressure.
(1.72 Tons by Burrard's conversion is only 4,660 psi and most period reported numbers for 1 1/8 oz. 3 Drams Bulk Smokeless were 6000 - 8000 psi)
As you know, Parker Bros. cut the 20g chambers shorter than 2 1/2". I had a 1924 20g with 2 3/8" chambers. With AA 7/8 oz target loads the case mouth was feathered and recoil was WICKED.
As often discussed, lengthening forcing cones WILL diminish chamber pressure, and modern 20g nominally 2 3/4" shells are often shorter than 2 3/4".
![[Linked Image from photos.smugmug.com]](https://photos.smugmug.com/Shotshells-and-pressures/Shotshells/i-LsPgXcJ/0/KwWXzBtPKrQ3f3wJsWs66pZDz2CfHmN6kFFs23ghk/L/Shells1-L.jpg)
