The Mk22 "Hush Puppy" unlocks at level 22 and appears as the Desert Eagle Mark XIX (Deagle Pistol).
This weapon is a powerful semi-automatic pistol with a special slide lock mechanism, drastically reducing recoil, and potential noise when the weapon is silenced. The only drawback to this is that you have to rack the slide after each shot to chamber the next round, making the rate of fire dramatically slow. The pistol includes a unique silencer, and a special "tranquilizer dart" ammo type, which poisons enemies on hit.
