This Carl Gustaf M/45 also known as the Swedish K submachine gun was found in 2008 in Sweden by a group during a chainsaw training exercise.
Caliber: 9x19mm Luger/Parabellum/NATO
Weight: 3.43 kg empty
Length (stock closed/open): 552 / 806 mm
Barrel length: 203 mm
Rate of fire: 600 rounds per minute
Magazine capacity: 36 or 50 rounds
Effective range: 200 meters
The M/45 is a simple blowback gun the fires from an open bolt. It was created in 1945 and used by the Swedish military.
Here’s one not stuck in a tree:
And here’s some in use by Swedish competition shooters. They’ve been modified with brass catchers and sight hoods.
Video [14:55] https://youtu.be/lYGthBDzt8o?si=
I wonder if the pipe rifle in fallout 4 was inspired by this.