Sentry Guns are automated turrets outfitted with dual chain guns in order to combat enemy infantry and light vehicles. Though not equipped to deal with more heavily armored units, the Sentry Gun works well as a preliminary frontline defense.
Sentry Guns are the Soviets' countermeasure against scouts and small infantry skirmishes in the early game. Cheap and reliable, it has little problem dealing with Tier 1 infantry and even vehicle threats (except heavier main battle tanks like the Cavalier Tank). In the late game however, its role is quickly outshined by the more powerful Battle Bunkers and Tesla Coils, so the Sentry Gun becomes a backup defense that the Soviet general can deploy in little time when necessary (such as a power sabotage that renders most Soviet defenses offline). Nonetheless, its effectiveness against infantry can be a lifesaver against infantry raids (especially Navy SEALs attempting to plant C4 to important structures) and spies (the general has to manually target them however).