The Desert Express .50 is a gas powered pistol in Syphon Filter: Omega Strain, Syphon Filter: Dark Mirror and Syphon Filter: Logan's Shadow.
"The Desert pistols are the only gas - operated pistols in service and the .50 caliber is the most powerful. The Desert Express is often referred to as a hand cannon due to its size and the power of the round. Only users experienced with great recoil forces will be able to handle this weapon effectively. Anyone hit within its effective range will be knocked to the ground instantly even if they are wearing heavy armour."
This is one of the later weapon unlocks due to the player having to earn Logan's commendation to acquire it. There are a few benefits to this weapon: it can kill almost any individual with an instant hit; but to balance its power, ammunition is rather rare. In order to get it early on (but temporarily), one can get it from the magic box trick.