The Sentry Gun was an automated Soviet defense mechanism that was introduced during Third World War and remained in service during the Psychic Dominator Disaster. It was a robotic gun turret that used a rudimentary AI to fire on enemy soldiers and leaves friendly ones unharmed. It was built as a Russian equivalent to the Allied pill box.
Despite the Soviet Union being known for its nigh-obsessive propensity for excess firepower, the removal of the unreasonably deadly flame tower from Second World War arsenal proved that the Russian military had some ability to limit itself. Realizing through the trials of the Second World War that the flame-thrower mounted watchtowers were as dangerous to Soviet troops as they were to Allied ones, the Red Army quickly sought out a suitable base defense to act as a safer replacement.
Utilizing AI technology, Soviet military scientists developed the sentry gun, an automated gun turret that fired 12.7 millimeter bullets at a blindingly rapid rate. Using a simple camera "eye" and early "friend/foe" programming, the turrets were able to detect who was a member of the Red Army and who was not via visual approximation, essentially telling the machine who to shoot and who to avoid shooting at. While designed mainly for killing infantry, something which they excelled at, the sentry guns were also fairly effective against lightly armoured vehicles and even did reasonable damage to their heavily armoured brethren. In suitable numbers, the turrets could combat most basic threats.
Sentry guns had some weaknesses. Firstly, while they could easily traverse a full 360 degrees, their elevation mechanisms had a very limited range of motion, making it impossible for them to shoot down aerial targets. Secondly, because the turrets relied on visual accuity to determine enemies, they were unable to recognize disguised spies and will not fire upon them unless specifically commanded to do so.
In spite of being AI-controlled, they were vulnerable to manipulation from Yuri Prime.
Finally heavily armored units like heavy tanks will withstand their firepower long enough to kill the Sentry guns.