Welcome to the forum JK. The Russian round seems to be over .3 shorter than the -06. I'm not too familiar with the Mosin, but others will give more enlightened opinions soon I'm sure. This is a great forum for that!
Another issue you might have is feeding, since the 7.62x54mmR is rimmed and the .30-06 is not. I'm sure it's doable, it's just a matter of how easy it would be and if it would be worth it.
The M-N magazine lacks about 1/8" of allowing the 06 case to enter. A 308 Winchester shell will enter the magazine with some room to spare. Probably could be modified to work but then the receiver would need modification also. Would be more work than its worth. Would suspect a 303 British round could be made to work much easier considering its rimmed and somewhat general shape of the 7.62x54R round.
Conversion to .30-06 was done to quite a few MNs, decades back when they first came on the surplus market. The common advice for these guns today is -- don't fire them. The consensus seems to be among sources I have read that the pressure difference between the 7.62x54r and the .30-06 is too great to be comfortable with in this action.
Is that true? Heck if I know, but i am sure of something -- the 7.62x54r is nothing to sneeze at, and comes darned close to being as useful as the .30-06, and the action works like a charm when chambered for it.
IMO, the MN is as perfectly what it was designed to be as it can be, just as it is. As-is, they are darned nice rifles in many ways.
Before you spend all the money to convert to a 30-06 consider how much ammo or reloading supplies you could come up with for the same price. I had my 91/30's bolt handle bent down and the receiver drilled and tapped for a scope for under $100.00. Less than another $100 was spent in procuring reloading supplies. Rifle currently shoots about 6/10" at 100 yards from the bench for 5 shots. Total for the rifle, the scoping, and the reloading supplies was under $280.00. I believe a conversion would cost a whole lot more than that. Just a different point of view.
First and foremost, welcome to ShootersForum! Rules are simple: Be nice and join in!
As this is your first post, I will let you know that there is a great wealth of knowledge to be gained on these pages. The many years of first-hand experience from the gentlemen who are members of this forum never ceases to amaze me. If you heed their words you will learn from someone else's mistakes and misadventures!
Pisgah and Oldbull about covered it: You could convert an old 91/30 to shoot the '06 cartridge, but you would spend a lot to gain very little. Shoot it "as is" and enjoy it. Mine has an after-market stock and scope mount that have turned it into a minute-of-deer rifle out to 200 yards, or more. To my way of thinking, there's no need to improve on that.
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