>>>In the bottom photo is a new American Remington case and it is setting over a 1/4 inch below the lip of the gauge. This means the shoulder of the case is a 1/4 of an inch shorter than standard Britishicon military ammunition.<<<
From the photo 1/4" is a bit of exaggeration. In any case (to make a terrible pun) I don't an O ring the size of a B-36 tire will correct it.