How does bore evacuator (fume extractor) work? 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. The evacuator is a reservoir that holds the super-heated, high-pressure propellant gases produced by the firing of a shell, then releases t