The book doesn't realy cover their actions just that the USA sent some volunteers, and of course he mentions the Brewstr B-239's that arrive at the end of the wnter war, just before the truce. Just read an instance where a Finn officer uses his pistol to provide cover to a fellow oficer (the officers lead from the front and suffered heavy losses.) who was attempting to take out a russian T-28 tank with a roll of dynamite sticks. The one stands and shoots at the view slits while the other crawls toward the tank, dropping into cover, the shooter continues to drop, reload and then stand again each time emptying three clips before the tank retreats never giving dynamie guy a chance to throw. They both had cast iron ones...
So many of the Finns were essentially well lead militia and paramilitary (police border, guards) and it was a hunting skiing culture that heavily favored light automatic weapons.