"The reason for an open bolt is for cooling."
Yep. With an automatic weapon you can heat the action up quickly enough to cook off rounds, and the firing from an open bolt helps prevent that.
There have been numerous full/semi autos over the years (primarily ones that fire rifle cartridges) that fire from an open bolt during automatic fire, but when switched to semi-automatic mode fire from a closed bolt.
The Johnson Light Machine gun comes immediately to mind.
"The gift which I am sending you is called a dog, and is in fact the most precious and valuable possession of mankind" -Theodorus Gaza
Baby Jesus cries when the fat redneck doesn't have military-grade firepower.