|Technician: Engineer||Tier 1|
|Basic (1 pt): The cost of deploying a sentry gun is reduced by 5%.|
|Ace (3 pt): Your sentry guns gain a protective shield.|
Third Law basic is rather straight-forward: the ammunition cost of deploying a sentry gun is reduced from its default of 30% of your ammo to 25% of your ammo. It also shifts the minimum ammo needed to deploy a sentry gun from 30% of your maximum ammo to 25% of your maximum ammo.
Note that due to oddities in the coding for ammo removal when a sentry gun is deployed, the ammo cost of deploying a sentry gun is 25% of your current maximum ammo rather than total maximum ammo. It still prevents placement if you have less than 25% of your total maximum ammo on hand, however. This does not affect how much ammo is held by the sentry gun.
Third Law aced adds armor plating to the sentry gun which covers the entire front and right-hand side of the gun, as well as an angled plate which covers about half of the left-hand side. Any bullets, be they from cops or from allies while the Friendly Fire mutator is active, deal no damage whatsoever to the sentry gun if they hit one of the armored plates. Even sniper rifles cannot pierce this armor. Note that while the gun barrel may appear to be left exposed by a hole in the front plate (and on silenced sentry guns, the suppressor sticks out of the front armor plating entirely), the area is still counted as armored and takes no damage when hit.
- Unless you only intend to use the Engineer tree in order to gain access to Jack Of All Trades, Third Law aced is a must-have for a build that makes any use of sentry guns. The bulletproof shield lets their otherwise meager health pool last much longer in any given situation, as they will be effectively invulnerable to damage from a target they are facing and firing at.
- While the front and right side of the sentry gun becomes invulnerable once this skill is aced, keep in mind that the angled plate on the left gives little protection from fire coming straight at the left side. In fact, while the top, bottom, and back of the sentry gun are completely armorless, this gap on the left side is the largest weak point on the gun - sustained fire from a single enemy back and to the left of the gun can still bring it down in a matter of seconds, especially on higher difficulties. Do your best to minimize chances of the gun being surrounded or exposing its left flank.
- Make sure to place sentry guns such that they start out facing the direction cops will likely appear from. When there is no target in sight, sentry guns will turn to the direction they faced when they were deployed, so make sure this default position is towards a point of entry for enemies. It reduces the chances of enforcers catching the sentry off-guard and slipping a couple shots past its shield while it turns to face them.
- Try to place sentry guns in corners or small nooks. This prevents cops from surrounding the gun and exposing the left flank to their comrades. It may reduce the size of the area which the sentry can guard, however.
- The name of the skill is a reference to Isaac Asimov's third law of robotics, which states that a robot must try to keep itself from being harmed, so long as doing so does not interfere with the first law (do not harm humans) or second law (follow orders given by humans). While its aced version remains more or less faithful to its namesake up to the point of letting a sentry gun protect itself better with a bulletproof shield, it also indirectly violates the law and the two preceding it at the same time by allowing the sentry to harm and kill humans.