This article is from the Paintball FAQ, by Paul "Stryder" Kloehn pkloehn@frymulti.com.
A closed bolt gun is one that is in the "ready to fire" position with
the bolt closed. That is, the gun is ready to shoot when the front of
the bolt is secured into the "chamber," or the breech end of the barrel.
When the trigger is pulled, a valve is opened that allows a burst of CO2
through the bolt to propel the ball down the barrel. Closed bolt guns
re-cock themselves (or are pumped, in the case of pump guns) by opening
enough to allow another ball to fall into the chamber, then re-closing,
ready to fire again. Autocockers and pump guns all use the closed bolt
design.
An open bolt gun is one that is in the "ready to fire" position with the
bolt in the open position, ready to slam closed when the trigger is
pulled. With most open bolt guns, when the trigger is pulled, the bolt
slams forward sealing the ball in the breech of the barrel. A burst of
CO2 is then released that propels the ball down the barrel. A spring
returns the bolt and re-cocks the bolt. Most, but not all, semi auto
paintball guns are open bolt guns.
 
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