In MW3 Zombies, you can activate Deadbolt Turrets by using different types of Ammo Mod Circuit Boards. I've used these turrets against special zombies in the medium and high-threat zones, so here are some methods you can follow to obtain the circuit boards easily.
Where to find Ammo Mod Circuit Boards in MW3 Zombies
The easiest way to obtain Ammo Mod Circuit Boards is from Aether Loot Caches located all around the map. You can find these caches easily after consuming the Death Perception Perk-A-Cola as it allows your operator to identify loot boxes through walls and structures.
Similarly, Ammo Mod Circuit Boards can be obtained from Reward Rifts after completing different contracts. I recommend completing contracts like Deliver Cargo, Aether Extraction, Spore Control, Bounty, and HVT. Ideally, you want to complete these contracts in the medium threat zone, as the rewards are much better.
Another method to obtain Ammo Mod Circuit Boards requires players to cleanse Aether Nests. After clearing out all the cysts in an Aether Nest, open the loot caches to collect the items. Similarly, you can infiltrate Mercenary Camps by eliminating all the AI combatants to collect the loot from the caches. Most often I've found tons of Ammo Mod Circuit Boards inside the loot caches at Mercenary Camps and Aether Nests.
Likewise, Infested Strongholds and Mercenary Strongholds are top locations where you can find items like Ammo Mod Circuit Boards, Perk-A-Colas, 3-Plate Vests, Killstreaks Ammunition Mods, Weapons, and other equipment. I've tested all of these methods in-game, and these are the easiest methods to get the best loot.
How to use Ammo Mod Circuit Boards in MW3 Zombies
To use an Ammo Mod Circuit Board, visit any of the Deadbolt Turrets located around the map. I suggest visiting medium and high-threat zones and activating the turrets at these locations. For instance, I like to visit the red zone (high-threat) after obtaining an Ammo Mod Circuit Board and activating Deadbolt Turrets close to special enemies like Mega Abominations. The Deadbolt Turrets fire automatically at undead hostiles, so essentially you can farm top-tier loot from higher threat zones by simply collecting circuit boards.
Wondering how to use the grenade launcher in MW3? Check out How to destroy enemy equipment with the RGL-80 in MW3 on Pro Game Guides.