I'm certainly no gun nut, but I chanced upon this marvel on YT, admiring what a great leap forward it was for the Allies. The main ideas here are: it's inconspicuous, breaks down easily, and has the most effective built-in silencer the world had ever seen, at that point. Evidently a fired bullet sounded no louder than a car door shutting from close range, and from distance, in city and crowd environments, it was effectively impossible to hear. Indeed, solving the multi-problem of guns creating noise in three significant ways was the major breakthrough.
So all this opened up situations in which The Resistance could take out SS and Gestapo officers with high chances of not being detected. The two limitations being: 1) it needed to be used from shortish range, and 2) the system of rubber sound barriers inside the barrel wore out rather quickly.
More on WP:
https://en.wikipedia.org/wiki/Welrod
Here's the nicely-narrated 10min video I watched:
https://www.youtube.com/watch?v=1R1kKHYZiQE
And this 2min video dives closely in to the mechanics:
https://www.youtube.com/watch?v=k-2l27S9hrE
Hah! Yeah, I saw the VP9 when I was browsing for images. Certainly looks like a gussied-up version of the Welrod.