Don't forget the K31's typically have a .304"-.306" groove diameter barrels for that slightly smaller 7.5 mm bullet.
Wait, what? Where did this information come from? I haven't seen anyone say they had a barrel slug out at .304", it's usually .307" and occasionallly .306". According to the Swiss military, GP11 uses a bullet up to .3087".
Edited to correct a misremembered measurement.