The American and British "system" of shotshell lengths is confussing. There are 2,0, 2 1/4, (2 3/8?), 2 1/2, 2 9/16, 2 5/8, 2 3/4, 2 7/8. It is so much simpler with just 65, 67, 70 mm.
Then add actual chamber lenghts -- it is widely reported that USA makers once found that 1/8 inch shorter chambers than fired hulls gave tighter patters. Then add the modern tendency to increace forcing cone lengths in old guns from those essential with card and fiber wads.
Most of my old doubles have 65 or 67 mm chambers and short forcing cones. I shoot 65 and 67 mm ammo in them rather interchangably, as "allowed" by CIP regs. I DO NOT shoot American-made ammo with actual 70 mm hulls and near SAAMI chamber pressures in any of them.
I don't even shoot low pressure handloads in true 70 mm hulls in any of the 65 mm chambers. Reason? More recoil, which can only be higher velocity, which can only be result of increased chamber pressure, which can only be because of additional 5,0 mm of hull in short forcing cone. I do not find this with 65 vs 67 mm hulls.
Some shooters get the short (about 1.0 cm) forcing cones lenghtened to 3-4 cm. They claim it lets them safely shoot 70 mm hulled ammo in these guns, without lengthening the chambers.
Take any simple statements about chamber vs fired hull lenght with ample salt, including Tom Bells qualified statement.
Niklas
