You will certainly find significant variations when it comes to 16ga bore diameter. However, on most European guns--and it's certainly true of German guns, although I can't say positively when they started the practice--if the bore left the factory with a significant deviation from the 16ga standard, then the gun will carry a 2nd mark indicating the nonstandard bore size. Example: the barrel flats on my Sauer 20ga are marked 20 in a circle. However, right next to the 20 in a circle is a 19. That indicates that my gun is significantly overbored from the 20ga standard of .615, to the tune of about .010. And measuring the bores indicates that is indeed the case.
It's the same system the Brits used, Greg--with many 12ga guns carrying numbers like 13, 13/1, 12/1, to indicate that the bores are either underbored or overbored from the standard .729. And to keep you from thinking that the gun is out of proof when you measure the bores and come out significantly tighter or more open.
