The M9 SD is the silenced version of the M9. Due to its near silence, it's one of the best sidearms for killing zombies safely. As a downside, the only ammunition it can use is M9SD Mag., making it difficult to keep loaded.
Another issue with the weapon is its low damage. It takes five shots to kill a zombie without a head shot, so ammo is not only hard to find but easy to waste.