I see! So the Llahti 20mm Anti-Tank Rifle, if it was widely available during the Winter War, could definitely be a game changer.
I'm trying to write a TL about a Winter War that goes in the favor of the Finns. So far, I have the following PoDs down to make this feasible:
1) Finland gears itself for war earlier on (beginning of the 1930's) and begins to import and produce more weapons. The military is given more funds and the conscription is set in place for potential war. This allows Finland to be better prepared and well equipped when the Soviets roll in to Finland.
Finland had fairly widescale conscription with some 2/3rd of annual male age class taken in already during 1930's with 350 days for privates and 440 days for officers and NCO's. This was supported by widescale refresher exercises and volunteer Suojeluskunta organization. I don't think there's realistically much room to improve here. Of course training might be improved, less conscript time used for work purposes instead of training etc, but looking at the European situation Finnish training was top notch.
As for defense expenditure, it was fairly high in European standards throughout 1920's and 1930's. However, one place to improve would be the period of Kamikaze spending in 1938-1939. Finland had low taxation and basically no foreign debt. If low taxation and low foreign debt wasn't seen as an economic priority not to be violated Finland could have had vastly better situation in number of items. Above all, US debt market was open for Finnish offers and Finland could have bought massive amount of equipment of US factories hungry for orders in addition to domestic economic mobilization.
Am I missing anything crucial?
Finland did not do economic wargaming during 1930's. As a result, Finnish foreign trade was virtually cut off from the world when Germany closed the Baltic sea. Even some modest preparations, such as leasing harbour space in Narvik, preparing the Petsamo for foreign trade and perhaps Lyngefjord as well, would have been a vast improvement.
Even more importantly, domestic production of food should have been improved. Finland could have been made self sufficient in food production, especially with domestic fertilizer production being a possibility.
But this all comes down to one important economic point: Finland had vast possibilities for economic improvement and industrialization in 1920's and 1930's and state taking a role in economic projects was in itself not seen as heresy. However, taxation was kept low and taking foreign debt was an aberration. Thus there was lack of capital for economic improvement.