But I repeat myself
Sherman Bell's study is about 1/3 way down here
https://docs.google.com/document/d/1...vwLYc-kGA/edit
His study was for 12g shells, and the average increase was
700 psi.
There is no data regarding the pressure increase in using 2 3/4" modern plastic case shells in short chambered 16g and 20g guns.
It has been established also that Parker cut the chambers slightly shorter than the nominal 12g 2 3/4", 16g 2 9/16" or 20g 2 1/2".
I had a 1924 20g Parker with chambers 2 3/8", and recoil was wicked with AA 2 3/4" 7/8 oz. target loads.
Also, 20g shells in general do run significantly higher pressures than 12g.
We are all free to do what we wish with that information, but not free to make stuff up.