If the bore is different, then they can't be the same. Somewhere on the horn you have to make up for the couple of thousandths in size - or put up with a step in the bore. My large bore Bach C has a medium sized bore (.453) where the bell starts but large bore at the second valve.. Bach obviously did that for some reason. I have given my best based on many years of paying attention. It is frustrating sometimes seeing BS repeated over and over again.