Yeah, a suppressed .22LR with subsonic ammunition can really be, for all intents and purposes, almost completely silent. At least, as far as the sound of the discharge goes. The action cycling on a semiauto would still make a sound. I suppose if you had a suppressed bolt-action or single-shot gun, even that could be eliminated.
Of course, for the purpose of your story, you would have to remain within the limitations of the very small and relatively low-powered .22LR cartridge in order to keep things plausible. Probably the only circumstance under which you could expect a “silent” kill with a .22 is a careful headshot, since anywhere else stands a pretty good chance of leaving the victim conscious and able to act (and yell), for at least a short time. Don’t overstate the ballistics or terminal performance and you should be fine.
On the other hand, you might want to consider how much you expect your audience to know, and how readily they’ll believe you if you go on about a perfectly silenced bolt-action .22LR. It might seem more plausible to your readers if you have the assassin use a suppressed 9mm and simply take advantage of a covering noise, like nearby traffic. But, if you must have absolute silence, it is at least physically possible.
(Incidentally, I would just love to pick up a silenced Walther P22 for plinking. Unfortunately, it’s technically a misdemeanor to fire a shot through a suppressor in my state. It’s perfectly legal for me to buy and own one, and they sell them at pretty much every gun store in the state (on a wink-wink-nudge-nudge only-fire-this-in-Oregon basis), but I’m not really comfortable being a scofflaw. It’s a silly law, though. I should phone my representative.)
I have to take exception to that. I’ve tried firing my .45-caliber M1911 pistol without any hearing protection on occasion, and the only thing I can say is good god that thing is loud. Each time it has produced only a sudden shock of pain in my ear area, leaving me to subsequently question my sanity in trying the experiment as my hearing slowly returns and the echoes fade away across the valley.
Okay, it’s definitely quieter than my 12-gauge, and it won’t be as loud to someone standing a good distance away, or across several walls, and I might be exaggerating just a little about the echoing through the valley thing, but “not particularly loud” is the last phrase I’d use to describe it.
ETA: And, of course, there are Google ads for “homemade silencers” at the bottom of the page. :smack: