Welcome!
I think it is 1 in 10, but I'm sure someone else with more knowledge than me will be able to confirm.
Incidentally, my Parker-Hale shoots lighter bullets (58grn and 60grn) absolutely fine, and 100grn absolutely fine, but anything in between that (such as 80 or 85grn) it hates!
Funny how these old rifles have their little idiosyncrasies!