Sentry Guns are found in Random Boxes at a quite a high chance, at times numerous sentries can be found on a typical match. You place them by pressing space and clicking on the sentry gun icon above. They are deployed where your character is standing. Once you place them you cannot change their position, so use them wisely. They can also be bought from the in-game pause menu.
There are 2 types of sentry guns;
The GECAL 50 has very low damage, a high rate of fire, and a clip of 400. The GECAL is not very efficient, if you are lucky you can get it to last about 1 round if placed well. It also has troubles with even the weakest zombies as it takes about 30 GECAL shots to kill one swarm zombie. The GECAL costs $5,000 from the in-game pause menu and it does about 31 damage.
ADWS (Area Denial Weapon System)Edit
The ADWS is far more powerful than the GECAL weapon system, as the ADWS shoots every second preserving its low mag of 250. However because of its slow fire rate, it is only really useful on the slow large zombies (shadows, butchers, mamushkas, and devastators). An ADWS can last up to 3 rounds when placed efficienly. Instead of having a fixed damage, the ADWS takes away 20-30 percent of a zombies health, making it epic in killing butchers and other high power zombies but terrible for sprinters and swarmers. The ADWS costs $10,000 from the in game pause menu.
Sentry guns can be placed around the map wherever you want and whenever you want, you can even place them when you are down! A good idea is to plant these near zombie spawn points or somewhere nearby you in singleplayer. If you plant it near the zombies spawn, the ammo will go quick but it is a good place to put on multiplayer as they can kill the zombies before other people kill them, which will make your kill count at the end of the match higher. Placing it near you is a good idea (especially on Singleplayer) so it can protect you and you can save ammo as they damage the zombies for you. All the kills from the sentry guns that you set up will go to your kill count at the end of the game.
An ADWS placed on top of a purge nest can (slowly) kill the nest by itself. One possible strategy when becoming overrun in purge is to use a cryo grenade to create an opening to drop the ADWS, then continue kiting zombies until the nest is dead.