The British Army is training troops with a version of a computer program called Half-life which allows up to eight soldiers to work as a unit moving through a simulated environment.
Like most shoot 'em ups the soldiers fire guns, throw grenades and even plant plastic explosives using a mouse and keyboard.
Often the Army pits two teams of four against each other, with one side playing terrorists and the other counter-insurgent troops. The game allows them to try out new weapons and tactics.
Major Bruce Pennell, of the Army's Logistics Corps, told the BBC that the game had worked well.
The software has been stripped of its fantasy elements by makers QinetiQ to drive home the consequences of making a mistake in real combat.
QinetiQ spokesman Chris Morris said that in an ordinary game a player's 'health' goes down by a certain number of percentage points if they are hit. In the Army version one bullet means death.
The Ministry of Defence is currently evaluating the system.
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