7,8 mm mark stands for 8x57JR, when 7,9 mm stands for 8x57JRS.
Your Nimrod drilling originally made for 8x57JR.
Gentlemen, as usual Geno is correct and I,
a. have too many guns and can't keep track of them, or
b. don't get to use them enough.
Geno's post made me nervous as I had bought 200 rounds of ammo,, sighted in the gun and took it hunting 5 years ago. I went and checked my records and found that when I was younger (5 years ago) I was astute enough to actually slug the bore and purchase the correct ammo, 8x57 JR (.318).
Thank you again Geno, for reminding me that sometimes I'm not as smart as I used to think I was.