Max bullet diameter for British .303?

[spinecracker]

I recently slugged the bore on my 2-grooved 1941 Long Branch No.4 Mk.1, and it came out to a groove diameter of 0.315" and a bore (lands) diameter of 0.303". I have been using Sierra MatchKing .311 174gr HPBT with 38.3gr of Varget, Prvi brass and CCI large rifle primers. 5 round group size with this load at 100 yards is a consistent 1". I am considering seeing if a larger buttlet might tighten up group size a little, so I will be trying out the Hornady 0.312" 174gr BTHP bullets, but I was wondering what would be the maximum safe bullet diameter for my situation (e.g. the minimum difference between groove diameter and bullet diameter)?

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