Larry;
I think we are actually making progress. I think you have made an accurate assesment of the situation Fergus encountered. As I recall his acount the gun was a very early one & the cone was so short as to be almost a step.
It is also noted that 65mm & 2½" are not mathamaticaly identical. 2½ = 63.5mm. I have not had opportunity to measure a large number of British guns but from what I read I believe that many, if not most nominal 2½" British guns did have the 65mm chambers which are about 2 9/16". Now if this early chamber were a true 2½" (It is my understanding some were) & if that cone was on the order of just a 30°-45° chamfer at the end, then even a 67.5mm hull could have entered the bore proper, not just a portion of the cone. I highly suspect this was somewhat the condition Fergus encountered. This could indeed create a check to the early movement of the shot causing a rapid build of pressure. I can also see a sudden release taking place which would cause this high pressure to give an enhanced velocity to the charge & in fact the cutting off of the portion of the hull in the bore may have provided this release. This enhanced velocity would then create the extra recoil. It is though quite possible that with a different set of conditions A pressure spike can occur but this sudden release not happen & there be no increase in either velocity or recoil. So yes Larry, any abnormal increase in recoil should most definitely be immediately & thoroughly investigated for cause, But the point I have been trying to instil is "If" it does not occur, one cannot automatically determine no abnormal pressure has occured. To load a longer shell in a shorter chamber established guidelines must be adhered to & Recoil is not a reliable indicator of Maximum Peak Pressure.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra