Well, why do you want to convert them, first of all? You can get new production 7.62x54R (Wolf comes to mind, non-corrosive, inexpensive, and consistent enough to hunt with).
Now, don't try this, but if the bullets are the same (.30cal 7.62), and their weights are comparable (fall in the same range, I.e., 7.62x54R let's say is 200 grain, where your average 7.62x51 is 180 (I THINK, not sure on grain #s so don't go on these), couldn't you come up with a shim to prevent the shorter 7.62x51 from falling in to far? Would that work?(asking those who know more). How awful would it be for the bullet to have that little gap before engaging the rifling/barrel?
The proper way would be to rebarrel those guns into the x51... You can see if they'd feed by just trying to chamber them as is for now.
Just throwing ideas out there... check brownells or google replacement barrels for them.
(easiest is just to get 7.62x54R...
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