The thing about guns, bollocks or lack of them aside, is that the standard practice for many years, confusingly enough, has been to express the calibre (as in basal diameter ) of a bullet or projectile,as a decimal of an Imperial inch !! Thus we have .30 Springfield, . 303 British, .308 Winchester,.408 Gibbs, .22 Winchester Magnum Rimfire, .45 ACP, etc.etc. So decimalisation of the inch has been around for a long time ! My favourite is .375 Wetherby Magnum, but then I don"t have any Nuts, do I ?? Only one way to find out, Liz !!!