65mm = 2.559" (2 9/16" =2.5625")
67.5mm = 2.6575" (2 5/8"=2.625")(2 21/32=2.65625")
70mm = 2.7559" (2 3/4"=2.750"))
Tatham #7 ran 291/oz so 1 1/8oz = 327 pellets.
Rt bbl was shooting 67% & left bbl 77%, 70%+ patterns are not a modern invention.
DuPont bulk smokeless, which is the powder which would have been used here, no other would have used that wt, weighed appproximately half an equal measure of Black powder. This was then essentially a 3DE-1 1/8oz #7 load (3D BP=82grs). This was "Not" a particularly stout load for a 7lb 9oz 12ga even in 1901.
It is my understanding from both Greener & Burrard that a nominal "2½" British gun has chambers which actually measure 2 9/16" (65mm) or in some cases 2 5/8" (66.67mm) & from Burrard that virtually all fold crimp shells sold after their introduction were loaded in cases longer than the chambers.
I have no idea how old Bell is, but suspect this was well settled prior to his Birth.
I measured a couple of fired US 2 3/4" hulls & they went to about 69mm The two important items for this discussion were both answered by the British work on this which was reported by Burrard in the above post.
#1- The "Loaded Length" of the shell should be shorter than the chamber so the crimp is free to begin opening un-inhibited.
#2- The "Load" inside should be compatible to the guns design.
A nominal 2½" gun with 3 ton proof should never be used with modern Regular SAAMI spec 2 3/4" shells.
"IF Both" of the above criteria are met it really doesn't matter if the actual hull length is 65mm, 67.5mm or 70mm in any gun having a cone length of at least 3/8" (9.5mm). I would never, ever attempt using a longer hull in a gun having a "Step" at end of chamber, but these are very, very seldom encountered.
Note also I do not consider the "Low Recoil" shells as "Regular".
