The gun is in its ready to be fired state when the gun is under pressure.
Under pressure means that the gun has some sort of fuel hooked up to the back of it. It doesn't matter of this fuel is C02 or Oxygen or Nitrogen, the principles of the marker will function the same no matter what kind of fuel you use.
After the gun is under pressure, the Hammer which is like a piston inside of a the marker is pull back and put under tension against a spring, the gun is held into place by a wedge shaped piece of metal called a sear. The sear fits in to a cleft in the bottom of the hammer.
The hammer is connected usually by pin or bar to a another cylinder called the bolt.
The bolt is the part of the system which is actually in contact with with the paintball. The bolt's main jobs are to regulate the loading of paintballs into the chamber and to make sure that the C02 is directed at the paintball pushing it out of the marker.
When the trigger is pulled on the marker the front of the sear is pushed upwards, thus pushing the bottom of the sear down and allowing the bolt to move freely. The spring which was putting tension on the hammer forces it to move forwards.
As the hammer moves forward it his a pin in a valve in the barrel and C02 is released which travels down the barrel and is focused by the bolt onto the paintball. Also as the hammer passes over the valve a small amount of C02 is also released from the rear of the valve which forces the hammer back into the cocked possession where it is ready to fire again.
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