After doing some research, I realized I was mistaken. I got my terminology mixed up. Apparently the nominal bore diameter for a K31 is .297-.298", so 7.60 mm would be a little above that. The groove dia. would be about .307" Which would still be a good if used with the right bullets. If the throat is not too worn, it should still be a good shooter.The given calibre for the ammunition being 7.5x55mm, and I have seen bores measured from 7.51mm (as new) up to this 7.60mm, so I assume it is worn by nearly 0.1mm. The 7.60 is 1/3 the price of the 7.51 and my pocket is very shallow as a pensioner so cheaper is good if it will cope. Match competition shooters will want the best but I am satisfied if it will be good enough not to embarrass me in public.