Sweden, unlike Finland, had money and time to develop a completely new service rifle.
The Automatgevär m/42 B is far more advanced than new barrels and stocks on an extant Mosin.
In Finland's defense, literally, they did as good a job as money, time and space would allow considering that The Soviet Union was being quite unneighborly to their east.
The Ljungman is one of only two rifles to reach general issue that use direct impingement. The other one is the French MAS Mle. 49.
You may notice that I did not include the AR. That's because it's not direct impingement, it is its own thing where the bolt carrier becomes the gas expansion chamber and the bolt becomes the piston.
The AG-42 uses a tilting bolt and there's no passage from the bolt carrier to the bolt at all.
The gas simply impinges directly on that face recessed inside that little cup and that sends the carrier back.
Disassembly instructions are here.