I have an old 12x12x9.3x72r German drilling (2 1/2" chamber). I have been using the Polywad Vintager shotshells. Recently I came across B&P on the internet. They make a lowe-recoil shotshells called F2 Sub Sound. Has anyone used them before. They are not exactly 2 1/2". They are 2.62" (62mm). What is the difference between 2 1/2" and 2.62". The difference looks small. Is it a new European standard? Can I use the 2.62" in the 2 1/2" chamber? Thanks.