Here are the 12ga minimum chamber dimensios from the above mentioned Dunlap book. This book is a 2nd edition carrying copyright dates of 1950 & 1963.
Dia at forward end of chamber .798"
Body taper .0044" (Inc) per inch
Dia at breech .8095"
Length breech to cone 2.6136"
Cone angle (per side) 5°
A minimum dia of .764" is then called out to a point in the cone a distance of 2.8079" from breech. This calculates out to an exact 5° cone angle.
A couple of points this 5° cone would have a length of .394" for a difference in chambe & borer of .069" (.798"-.729")
Chamber dimensions are given in this book for 12, 16, 20 & .410. The 12 is the only one given a chamber length shorter than the shell's nominal length.
Max length of the loaded 12ga sheel is 2.53" for a roll crimped shell & 2.41" for a fld crimp one.
It is also noted the other three sizes do call for a body taper of .005" (Inc) per inch.
