In this video Ian of Forgotten Weapons describes the Welrod Mk IIA covert assassination pistol developed during World War 2 by British Special Operations Executive (S.O.E.) to outfit special operations units as well as resistance units in German-occupied countries.
Its strange appearance is partially intentional, because without the magazine (which doubles as the grip) it looks somewhat like a bicycle pump and, more importantly, not like a gun. As Ian says in the video, if you’re stopped by German soldiers, you really don’t want them realizing that you have in your possession an assassination pistol.
Possibly the most interesting thing about it, from a mystery writer’s perspective, is that it actually achieves the sort of quietness that one sees in Hollywood depictions of silencers. (Normal sound suppressors, aka “silencers,” only reduce the unimaginably loud bang of the gun to an imaginably but very loud bang that, however, is not going to cause instant hearing damage.)
Ian says that about 14,000 were made. Further, they are still in occasional use by special forces, though special forces don’t particularly admit to it so this is slightly speculative. From the perspective of someone wanting to use a silenced gun in a murder mystery, these are sufficient numbers that one could reasonably find itself into the hands of an ordinary person.
Something else I find curious is that the manual for them says that their effective range is about 25 yards during the day and 7-8 yards at night. This is partially explained by the poor ergonomics and partially by having to use a slow bullet—a bullet traveling faster than the speed of sound will produce a supersonic boom when it hits the air and there’s nothing a silencer can do about that.
Now, nighttime shooting without special optics is always difficult, but I find it curious that in the daytime 25 yards is an easy shot with a bow and arrow, especially for a modern compound bow with carbon fiber arrows. A pistol is, of course, far easier to conceal than a bow and arrows are—to say nothing of being easier to carry—so I’m not trying to suggest that a bow and arrows would be better for the purpose than this gun. I merely find it interesting.