lancaster, in fact the actual bore diameter with the pre-1940 8.9mm stamp may have been as large as 8.999mm. It only sais a 8.9 would pass, the next size, a 9.0mm would not! In the days before the 1940 proof law that set minimum standards for the first time, barrelmakers were free to design their own dimensions they saw fitting. There was a belief in Suhl, minimum diameter barrels to be the more accurate. And, often larger-bore barrels as 9mm and 9.3mm were rifled with 6 grooves, the grooves double the width of the lands. Such rifling effectively increases the cross-section available to the bullet's passage.