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The very simple mechanism makes a Cheap Airsoft Gun cheap.  Before the trigger is pulled, a spring is drawn back to provide power to an air piston.  Gears assist in the pull.  When the spring is released, the piston then ejects the pellet from the gun.  Hence the name airsoft spring guns.  Electric or battery driven models have an advantage over the manuals in that they can be used as a semi-automatic.  A potential problem with spring guns is that they have a high degree of realism.  Most US states require a marker on the barrel to prevent any confusion.  Under no circumstances should a user remove the marker.