I noticed there is some interest towards Finnish crossbows on this forum. For the past few years I have studied several of these crossbows in various Finnish museums and also searched for archival material concerning their use. Naturally the following text can only scratch the surface of the topic but I would be happy to answer any question you may have.
There are no periodical pictural evidence concerning the use of crossbow in Finland, save for one drawn in late 18th century. All we have to work with are then the very few literature records, archaeological evidence and the museum collections.
The earliest archaeological crossbow records starts from the late 13th century when the first crossbow bolts appear. It was either the Danes or the Swedes who introduced this weapon to Finland. Also at the same time we will see arrowslits build on the first stone castles on Finnish soil. Collection of apostolic penitentiaries written down between years 1410 and 1526 and published in 2008 show that crossbows were fairly common in Finnish cities during the 15th century.
The castle ledgers beginning from year 1537 record two types of crossbows used by the military and held in the castles: horn and steel bow crossbows. The first ones were all imported but some of the latter ones were also manufactured in situ. Viipuri Province's taxation ledger from 1541 mentions a tribute paid by the people of Koivisto island to the Viipuri Castle: 21 crossbow stocks made out of Norway Maple. However this was quite special tribute and only used in that region.
According to my calculations, there are some thirty crossbows in Finnish museums. Most of them have only their stocks preserved while relative few have also kept the bow intact. The oldest stock is dated to year 1646, while the youngest one was made in 1772.
The construction of Finnish crossbows are very similar between individuals but Finnish researcher U.T. Sirelius managed to identify seven different subcategories based on minor details. In general most if not all of the stocks are made out of birch and decorated with inlays made out of forest reindeer's or moose's bone or antler. All of the museum examples I have seen have the bow made out of soft iron - steel laminate but on the other hand we also have historical information on wooden bows on crossbows.
Crossbow or bow was never a choice of weapon for big game in Finland; it was mostly used to catch otters, squirrels and rarely fish. Spear was used to kill the largest animals, i.e. bear, moose and reindeer. We have only one description on how the Finns used the crossbow, by Giuseppe Acerbi on his book Sweden, Finland and Lapland to the North Cape in the years 1798 and 1799:
In shooting the squirrel they employ, as has been intimated, a fort of blunt, pointless arrow, that they may kill the animal without injuring the skin: and what is deserving of being noticed, they do not take aim as we commonly do, by bringing the handle of the cross bow near the eye, but set it upon the belly; and yet by this method, which appears so awkward to us, they seldom or never miss hitting the object. The arrow is too valuable to be lost; for the moment it falls, it is picked up for another occasion.
Some crossbows from the collections of the Finnish National Museum as drawn by Sirelius in 1913.