A counter bored barrel could be a Finnish Capture Rifle. Check the left side of the barrel near the receiver for a "SA" within a rectangle. The Finns counter bored barrels when needed, and this ususally was from a barrel obstruction such as snow.
I would shoot it first and see what it does. As mentioned, you could probably buy andother M38 Mosin Nagant for the price of having a barrel put on this one.
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