And the best answer is to slug your barrel and be sure. I have read a lot about using pulled 7.62x 54 bullets for the .303. I beleive the bore is .312. Some of the bullets I have found for my AK measure anywhere between .310 all the way to a heavy .312. I size my cast bullets to .310
To find the optimum load you will have to do a lot of tinkering.

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By the way Jimb16 I like your quote and I use it often I hope you don't mind