The first 1/8" of a forcing cone is just a few thousandths smaller than the chamber dimension. In addition to that fact, the thickness of a plastic shell is much less than the paper shells that the old guns were made for. Don't worry, be happy. It is ridiculous to think that thousands of 2 5/8" chambers have been bored out to 2 3/4" over the years at great expense and wasted effort, with no advantage.