The Russian Mosin Nagant Forum

This has probably been beat-to-death, but why did the Finns open up the bore size on the M-39 after making a wicked accurate M-28/30 with the tighter bores.

I believe one of the reasons was to be able to utilize all the captured Russian ammo during the war(s).

I don't know how much Finnish history you've read, particularly as it relates to the Winter War. The soviets grabbed kids off the street in Moscow and conscripted them, sending them among other places over to Finland to fight. They didn't have winter gear. The weather during the winter war was among the coldest in Finnish history. So the Russian's built big bonfires at night to stay warm. That's one of the reasons why Simo Häyhä ended up with over 500 kills. So there was a lot of ammunition and rifles and other gear to pick up.

Finland was not a wealthy country. Money for guns and ammunition was very scarce. There were also logistic issues in getting supplies to the front. So they did what made the most sense at the time. They increased the bore size to accept the slightly larger Russian ammo. It doesn't matter how accurate your rifle is unless you have ammo that works in the rifle. With the larger bore, the supply chain became very short. Shoot some Russians. Pick up their ammo. Reload and shoot some more ...