The silenced .22 pistol is a .22LR round handgun with an integrated silencer. It's classified as a silent weapon, and can be used to attack targets without alerting nearby creatures or NPCs to the player's presence. It also has bonuses to critical hit chance and critical hit damage, making it an effective stealth weapon in tandem with sneak attack criticals. It is a holdout weapon, and as such can be snuck into places where the player's weapons are confiscated. Along with Li'l Devil and the Police pistol, the silenced .22 pistol is one of only three guns that can be used as holdout weapons regardless of the player's Sneak skill.
The silenced .22 pistol is the only Guns-skill weapon (outside of content from add-ons) which is classified as a normal holdout weapon, allowing you to sneak it into weapon-free areas regardless of your Sneak skill.
If this weapon is used to kill a member of a faction in one hit while being undetected it will not cause any Karma loss or infamy with that faction. Being detected will turn nearby people of the same faction hostile, and not killing the target in one hit will make the victim hostile, where afterwards killing will result in positive/negative Karma and infamy with that faction.