What I would do personally:
Take 10rnds of the surplus 7.62x54R in question, pull the bullets, weight the powder charges in each, find an average.
Take 10rnds of the same, shoot them over a chrony, find an average.
Look up load data for 7.62x54R and find a powder that will produce roughly that velocity, with roughly that powder charge weight, with roughly that bullet weight. You have now found a powder that should be close in burn rate.
Find published data for .303B for that commercial powder and the pulled bullet weight and load 10rnds but substituting the pulled powder and bullet. Reduce by 5-10% if wanting to be extra cautious.
Chrony those 10rnds and find an average; compare to published values.
Tweak the load, using a chrony, to your liking.
Keep in mind that different lot numbers of surplus ammo from the same factory may have slightly different powders. I'd rework the load from each new case of surplus 7.62x54R if they lot+factory numbers don't match.
I've found 7.62x54R bullets surplus bullets can weigh 147-151gr depending on who makes it (Chinese, Czech, Romanian, etc.). I load them all as if they're 150gr as the rifle wont be able to tell.
Take 10rnds of the surplus 7.62x54R in question, pull the bullets, weight the powder charges in each, find an average.
Take 10rnds of the same, shoot them over a chrony, find an average.
Look up load data for 7.62x54R and find a powder that will produce roughly that velocity, with roughly that powder charge weight, with roughly that bullet weight. You have now found a powder that should be close in burn rate.
Find published data for .303B for that commercial powder and the pulled bullet weight and load 10rnds but substituting the pulled powder and bullet. Reduce by 5-10% if wanting to be extra cautious.
Chrony those 10rnds and find an average; compare to published values.
Tweak the load, using a chrony, to your liking.
Keep in mind that different lot numbers of surplus ammo from the same factory may have slightly different powders. I'd rework the load from each new case of surplus 7.62x54R if they lot+factory numbers don't match.
I've found 7.62x54R bullets surplus bullets can weigh 147-151gr depending on who makes it (Chinese, Czech, Romanian, etc.). I load them all as if they're 150gr as the rifle wont be able to tell.
