The Assassina SPT-8098. Perfect for killing without a trace. Just slap a plastic bag over it to contain the GSR, load it with subsonic frangible .22s, blow away the target with a headshot at close range, wrap the barrel and any remaining cartridges in the bag, and toss it in a remote river. Subsonic rounds and a Nagant action allow a totally silent weapon, and cartridges can be made by pushing the bullet of a downloaded .22 WMR down until it is completely covered by the case. Frangible rounds mean that even if they find your barrel, they shouldn't be able to match the bullets to it, and the whole thing is made mostly of titanium (for weight) and can fit in your pocket. Developed by the Cordian Department of Foreign Affairs, Games and Theory (read; espionage) Bureau for use by Agents in sensitive assassination cases. Has no serial number, and technically never existed. Go ahead, ask the government. They'll deny ever hearing of such a weapon. Ugly as sin it might be, but damn if it isn't effective.
Interesting concept. As much as I love the .22, it isn't likely to kill someone, even at close range it tends to bounce off of bones. Maybe something larger but still quiet, .45 maybe? They've used it for quiet weapons for 70 years.
.22 to the head or heart has been a favorite of assassins and gangsters for a long time. The US special forces in WWII developed and used a suppressed .22 based on the Colt Woodsman, and the Mafia were fond of .22s for hit jobs. It's quite capable of killing at close range.
.45 can be silenced, but you need a huge fucking silencer to get it even close to .22 levels. Look at the DeLise Carbine's silencer.
The Socom .45 is a large weapon, but again it's highly effective (this is what I'm basing the idea on). Speaking of the DeLise, it is large, but that round was accurate in that weapon out to 400 yards. Even at subsonic speeds it could hold a lot of energy. It was just a suggestion is all. You could also do what the Russians do, large caliber, but half the shell itself is hollow with a bottle neck and plunger that blocks the bottleneck when the round fires so no gas escapes at all. No suppresser. Again, just an idea.
.45 can be silenced, but you need a huge fucking silencer to get it even close to .22 levels. Look at the DeLise Carbine's silencer.
I've already explained why silent ammo was not used for this weapon in some other comments. I'm too lazy to explain it again.