I have just been thru this problem. 65mm is not 2 1/2 in. It is about 2 5/8 which is about what most ammo is . 2 1/2 inch is 63.5 mm. I just ordered some 2 1/2 empties from Ballistic and to my frustration they are not 2 1/2 but marked 65mm. In my short chambered 12 ga gun they would hardly chamber. I called and complained about it and unfortunately they did not seem to understand the difference. As usual, I would not worry about shooting a "2 3/4 " in a 65 mm gun as it is close enough and likely the same chamber.
