The silenced .22 pistol is a .22LR round handgun with an integrated silencer. It's classified as a silent weapon, and will allow you to attack a target while sneaking without alerting other nearby non-player characters or creatures of your presence. The weapon's usefulness for stealth players is further complimented by its relatively high critical damage of 18, double the weapon's base damage. With the Better Criticals perk, a sneak attack critical from the silenced .22 pistol can cause (9+18*1.5)*2=72 points of damage.
Outside of stealth situations, the silenced .22 pistol is lacking, primarily because of its low base damage of 9, making it the second weakest Guns weapon behind the BB gun. This low damage is somewhat offset by the weapon's 3x critical chance multiplier.
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.