HITMAN 2: SILENT ASSASSIN UNLOCKABLES
Silenced ballers gun
Finish a level with a "Silent Assassin" rank.
Sawed-off shotgun
Finish a level with a "Silent Assassin" rank twice.
M4
Finish five levels with a "Silent Assassin" rank to get the M4. Alternately, complete the Shogun Showdown level with a "Silent Assassin" rank.
Silent Assassin ranks
You can still get a Silent Assassin rank on a mission if you are spotted. To get a "Silent Assassin" rank, there has to be nothing in the aggression bar. You can make sure of this by only using chloroform on guards and eliminating the target with just one shot.
Getting the .22SD pistol
The .22SD is well hidden in the castle level where you have to kill Hayamoto senior. It is one of the items that is displayed in his museum.