During it's earlier versions, Syphon Filter: The Omega Strain had various weapons and usable equipment which was not fully and "officially" included in the definitive version of the game, in addition, certain weapons had a cosmetic name change (such as the G11 - C11 example), many of these weapons were meant to be normally used in game but were later dropped. During Omega Strain's public Beta Version some of them could be normally accessed and used.
Nevertheless, numerous "Beta Weapons" were not completely removed from the release version of the game, and can be actually accessed and used in game (many of those are perfectly usable, even having their own weapon screen descriptions and sounds).
The main method of accessing these kind of weapons, this method can be used without the need of a cheat device (though there are certain limitations).
Using "Cheat Devices"
Certain commercial "cheat devices" such as Action Replay, Code Breaker or GameShark can be used to unlock every weapon and even making them available for normal selection in the pre-mission Equipment Selection menu.
Certain incomplete weapons which can be used were probably tested during internal Alpha versions (most likely intended for full presence in the last versions of the game), an example is the XM29 OICW. Some of these are incomplete and as such might lack certain properties (sound, specific name, 3D model, special effects, weapon screen description, etc).
Weapons which were essentially "complete" but were not included in the latest versions of the game (unfortunately), possibly due to the lack of extra awards/ratings (which, after being obtained, could consequently award these weapons for the players to normally use).
These can be used like any other normal weapon and most have their own complete Equipment Screen descriptions, sounds, etc.