And meanwhile, I even found out which use a chaingun has:
It is a formidable way to encounter "base killers", i.e. dragoons and tyrants when combined with a battle suit.
If you look at its
characteristics, you will realize that this one is capable of doing a damage of 1800 (!) healthpoints per clip, deploying this with a rate of 75hp of damage per second. So if you manage to chase an even fully healthy tyrant for a bit more than five secons, it will be a
dead tyrant. Save your stamina for this!
If you manage to corner a tyrant and it cannot kill you instantly whyever, it will be dead. Even if you do not corner it but it gets stuck because another alien or a human is in the way, it will fall aswell.
Meanwhile, the zybork in the chainsuit has fended of and killed many attacking big bugs, increasing human base safety. So if you are afraid in the night and feel insecure, get a man in the chainsuit guarding your home, and all will be well

PS: In any other case (=no battle suit), the chaingun is almost - useless.