A simple system keeps a tank crew safe

What’s that bump in the barrel of an Abrams tank 120 mm gun? It’s a bore evacuator. Some people call it a fume extractor. It works to pull harmful fumes from the gun barrel after firing, keeping the poisonous gasses from entering the crew cabin when the breech is opened to load the next shot. It’s a simple system actually. In that split second when the shell passes through the barrel, gasses are stuffed into the cavity through holes at massively high pressure. After the shell leaves the barrel, the pressure dramatically drops and the high pressure gas in the cavity vents into the barrel through outlet holes pointed downrange. As the gas rushes from the bore evacuator, it takes the gas left over in the barrel proper, too. It also helps when the breech is opened during this venting action, pulling fresh air from the tank’s cabin into the barrel as it is readied for the next shot.