Machinegun turrets are enemies in Ratchet & Clank encountered on Umbris and Oltanis, and also are used by the Unknown Thief on Siberius in Going Commando. They are static enemies that spray bullets in an arc in front of them, rotating from left to right and back again. On Umbris, they defended Qwark's headquarters during "Survive Qwark's gauntlet", and in the Gorda City Ruins on Oltanis, during "Search the destroyed city", activated when you were spotted by a blarg heavy interceptor. On the frozen base in Siberius they were only used by the Unknown Thief in his bossfight.
On Umbris, Machinegun turrets cannot easily be destroyed, as hitting one temporarily disables it once hit. The only reliable method is to use the morph-o-ray on them. They can be destroyed by normal damage but their health pool and invincibility whilst disabled means it's not worth the effort. Therefore, it is best to either shoot the Blaster and disable them briefly then move out of their range, or simply jump over their bullets. On Oltanis they can be destroyed. However, they have a faster turn rate and better range, meaning it is better to avoid them by avoiding the blarg heavy interceptor's spotlight, unless using the Tesla Claw to destroy them, or the Drone Device to defend against them.
In Going Commando they won't actually be registered as targets by the Seeker Gun, if they are even used by the Thief, for he is easily defeated before that happens. When used all four turrets focus on the center, leaving you free to stay near the outer area of the platform, and thus destroy them yourself.