First note I am not a Ballistician but have gleaned a few things over the years.
Much experimenting was done on this matter just prior to & immediately following WWII by the British upon adoption of the "Fold Crimp" shell. The general conclusion was that if the appropriate load for the earlier Roll crimp shell was placed into the longer hull & closed with a fold crimp, giving approximately the same length to the "Loaded" shell that neither ballistics nor pressure were significantly changed. The important factors were that "The Loaded Length" of the shell remained approximately the same, thus allowing ample room for the crimp to open without interference, only lapping into the cone a small amount. Normal chamber length for a 16ga is 2 9/16" while nominal 2 3/4" cases often run as much as 1/16" short, thus only giving a lap of about 1/8". The factors involved here are that pressure rises rapidly prior to actual moving of the charge. As soon as the charge moves then the "Burning Chamber" begins to enlarge & pressure starts to decrease. Of course it takes just a Wee amount time for this to occur, but generally by the time the charge is entering the cone pressure has peaked & started the downward side of the curve. Thus essentially what one would expect is not a "Rise" in pressure, but a slight retardation of the falling side. This would not be picked up on a pressure gun which records only the Peak or Max pressure but would require one which can measure the actual curve. The important factors are that first the load itself is suitable for the gun in question, next there is a space between the end of the "Loaded" shell & the end of chamber & finally that the gun does not have one of those very early type of chambers which have either a step or very short Bevel allowing the end of the case to actually lap up into the bore, but be a normal type cone of at least 3/8" length, usually some longer. I do not believe the actual gauge is a very significant factor if these other considerations are met.
Last edited by 2-piper; 06/13/09 12:31 PM.
