Erik: this is from the thread I linked above and I've lost track of some of the sources but believe this was from Researcher
All this being said there is a good body of evidence that back in those days chambers were held about 1/8 inch shorter than the shells for which they were intended. In the recently published book "The Parker Story" the Remington vintage specification sheets on pages 164 to 169 call for a chamber 1/8-inch shorter than the shell for which it is intended. Also in the 1930's there were a couple of articles in "The American Rifleman" (July 1936 and March 1938) on the virtue of short chambers. A recent issue of The Double Gun Journal carried an article on tests showing no significant increase in pressure from shooting shells in slightly short chambers. IMHO I don't much sweat that 1/8-inch in 12-gauge guns. On the other hand when one gets a 20-gauge chambered at 2 3/8-inch likely intended for 2 1/2-inch shells I do worry about folks firing 2 3/4-inch shells in such guns.
My usual load for that 20 is 3/4 oz at 1200, 1/8 less than the AA but the same fps. I wouldn't presume to attempt to explain the perceived difference (assuming that the thin plastic case entering the forcing cone contributes little to increased pressure) but there is a marked difference. Larry Brown suggested that if the forcing cone was very short the case mouth could even enter the bore of the brl.
A squad mate asked me if I was aware that recoil was bouncing the gun off my shoulder with the AAs. The gun is less than 6 # which may certainly add to the increased perceived recoil and I am very recoil sensitive (and with a hopeless flinch) from too much shooting with heavy loads in the early days of Sporting and some medical issues.
