A weapon could be found in the Ängsgär estate. But all the weapons I found did not count.
So I looked around for a video that showed a description of where to find something.
And lo and behold, there is a blatant divergence that may explain why I can not find the weapon.
In the video 2 weapon boxes are found in a large shed, but in my game there are no gun boxes in the same place (red circle in the screenshots).
Thus, the question arises whether the weapons boxes are at a different location each time the game is restarted or if this is a bug.
Since I found all the weapons boxes at this site, but did not count any of the weapons I found, I assume that it is a bug.