Odds are, you may know of the legendary Finnish KP/-31, and Ian with Forgotten Weapons not only has one of those but doubles down with a Swede M-37/39 because he’s like that.
Aimo Lahti crafted his Suomi submachine gun in the early 1920s in .32ACP, and it was adopted by the Finns in 9mm as the KP/-31 using either a 50-round coffin mag or a 71-round drum, and finally a 36-round double. The gun was well-liked and really successful– so much that the Soviets copied the drum for their PPD-40 and PPSh-41 after being on the receiving end of the Suomi’s 900 rpm burst during the white hell of the 1939-40 Winter War.
The guns were so popular the Finns kept them in service as late as the 1980s and they were widely used overseas.
Speaking of which…
As chronicled by Ian, Finland’s Baltic neighbor, Sweden, liked the Suomi so much they whistled up some of their own chambered in 9mm Browning Long– the same caliber as their Husqvarna longslides used. However, they soon switched to the more standard 9mm Para in their sub guns, as it was a bit spicier.