Until fairly recent times SAAMI specs for a chamber to be used with 2 3/4" 12 gauge shells called for a minimum length of less than 2 5/8". I am going on memory here but I seek to recall 2.618" with a maximum angle to the cone of 5 from bore axis. This would give a cone length to nominal chamber & bore dimensions of .400". This shorter length was not listed for any gauge other than 12 for some reason. Current listings do not show this shorter chamber, but a statement is made that chambers were formerly listed with shorter lengths & these chambers were still considered acceptable for use. A shell which opened to the full 2 3/4" length would be reduced at the mouth by 023" when fired in this minimum chamber.
Current minimum chamber diameter at the junction with cone is listed @ 12g = .798"; 16g = .732" & 20g = .685". Turn a plug to0 about .020" to no more than .025" under that chamber dimension for the gauge at hand & if it will enter to a depth of 2 3/4" don't fret the chamber. Be "Absolutely Certain" the load fired is compatible for use in the gun in question.
In a 2 5/8" chambered 12 gauge I would much prefer to fire a 1 1/8 oz load @ 1150 fps in a 2 3/4" shell than a 1 oz load @ 1330 fps in a 2 5/8" shell.
The total ballistics of the load are of far greater importance than actual chamber length as Long as there is clearance at the end of the shell for the crimp to open unretarded.
