The combat shotgun is a 12 gauge shotgun. It is used throughout the Syphon Filter trilogy and in Syphon Filter: The Omega Strain.
"This semi-automatic shotgun is designed for close range combat situations and is recoil operated. It has a magazine capacity of five shells and is equipped with a wide choke that causes the shot pellets to spread out and hit multiple targets with a single shot."
-Description from Omega Strain
In game uses
This gun can be obtained in almost every single level after the PHARCOM exposition center and is carried by many enemies. While it is extremely deadly against unarmored targets at close range, often being enough to take them down in a single shot, it fares very poorly against Flak Jacketed targets, as it is not capable of hitting fast enough to punch through without sustaining damage in the crossfire.
Syphon Filter 2 & 3
Only available in multiplayer, the Combat Shotgun is still as fun to play with as it was in the first game. Like its predecessor, however, it does not fare well against Flak Jackets.
Syphon Filter: The Omega Strain
This weapon is gained by being promoted to the rank of Intelligence Agent. Its fast firing rate and high capacity give it an edge rather than other shotguns, and it can be acquired early enough. The ALA army uses it in Carthage 1 and 2; it is later seen in the hands of Niculescu's security personnel. It is outclassed by other guns in its division, however, very shortly after it is unlocked.
It is unknown what type of shotgun this is based on, as thousands of different autoloading shotgun models could fit the description.