I have heard of it and not found any thing on it, It maybe a myth or an idea that never came to.
This is from wikipedia: \"Agar also added a cooling mechanism to the barrel, which consisted of a metal jacket through which air was forced to provide cooling. The air came from a turbine, which was powered by the same hand crank that was used to fire the weapon. This cooling air also tended to blow away any pieces of unburned paper from the cartridge that happened to be near the barrel. The rate of fire was also limited to 120 rounds per minute, which helped to prevent overheating.\"
Sounds like it used a turbine fan within the shroud that was powered by the same crank that fired the gun.
Here\'s the link to the above quote and article.
http://en.wikipedia.org/wiki/Agar_gun\'>http://en.wikipedia.org/wiki/Agar_gun
.