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How It Works - The VM68

Published Aug 1996
An Article by Mike Wallis

Ever tried to write down how something works? If I had you all here I could explain how a semi- auto works in about 15 seconds. I would have the benefit of a working example to show you. On paper I have to use words and diagrams and like an insurance claim the explanations can come out sounding ridiculous. Well, here goes and no questions please!

Pump Action Paintball guns require two actions to fire a paint pellet. 1-The gun must be cocked and loaded using the pump. 2- The trigger must be pulled to fire the gun. (Of course the auto trigger has blurred this a bit) Semi-Auto Paintball guns require only one action, the pulling of the trigger to fire. The most common semi available is the open bolt or blow back type. In this article I will explain the operation of this type in detail. I will use the VM68 / PMI-3 as an example because of its simple construction.

As can be seen from the diagram the PMI-3 has an upper and lower bolt joined by a pin so that they move backwards and forwards together. The upper bolt has the job of loading the paintballs into the barrel and porting the gas into the barrel when the gun is fired. The lower heavier bolt has the job of impacting against the gas valve to release the gas that fires the paintballs and re-cocks the action. The method of operation is very simple and is as follows.

The gun is cocked or made ready to fire by pulling the cocking handle rearwards until the trigger sear is engaged against the lower bolt locking it in place. This action compresses the mainspring that will drive the bolt assembly forwards when the trigger is depressed. Now that the bolt is rearwards a paintball can drop into the breach of the gun. The paintball gun is now ready to be fired.

When the trigger is depressed the trigger sear is pulled downwards unlocking the bolt assembly. The bolt assembly now moves forward as the compressed mainspring expands. The upper bolt now pushes the paintball into the barrel as the lower bolt strikes the gas valve. The valve opens releasing gas into a port that carries it up through the upper bolt to behind the paintball. The Paintball is now propelled out of the barrel by the expanding gas.

While this is happening gas is also being ported around the valve to the front face of the lower bolt assembly. This build up of pressure on the front face of the lower bolt now forces the bolt backwards with such force that it travels to its rearward position. The trigger sear now moves upwards under spring pressure to engage on the step on the lower bolt locking the whole assembly back in the cocked position. The gun is now ready to fire the next shot, only the trigger needs to be pulled to start the above sequence over again.

The VM-68 or PMI-3

To prevent the gun going full-auto ie. firing until the trigger is released. The mechanism is constructed so that the hook shaped part that the trigger is connected to is knocked off the sear when the bolt assembly moves forward. This allows the sear to move upwards under spring pressure to engage the bolt when it again reaches its rearward position.

Well, that’s it! I hope I managed to convey a plausible explanation and that no one is still scratching their heads in confusion. Maybe next time I can talk the editor into a pop up centre lift out with moving parts.(?) In my next article I will attempt to put back Mans' understanding of the closed bolt semi-auto (Autococker, Typhoon) by at least 20 years.