A bore evacuator is a device on the gun barrel of an armoured fighting vehicle which helps prevent poisonous propellant gases from venting back into the vehicle's fighting compartment when the gun breech is opened to load another round. Bore evacuators are most often used on large-calibre tank guns and self-propelled guns. Without bore evacuators, hot gases and other combustion residue leaks into the tank's interior, depleting oxygen levels and filling it with a foul odour that can easily induce nausea and distract the crewmen from their tasks.