Yes indeed .318, just a typo. I have edited the post. Thanks for catching that Raimey. I have mentioned it before but my barrel is actually marked 172.28 / 59. Except for that one silly little mm (remember those old cigarette Ads), a chamber cast revealed it to be dimensionally an 8x58R Sauer chamber As I said though the groove diameter per slug measures.321" with the bore going about .309", just shy of the next plug gauge size of 156.14 (.310").

Miller/TN
I Didn't Say Everything I Said, Yogi Berra