The pressure in the dump chamber of an automag is somewhere between 400psi to 550psi depending on the bolt used and the barrel length and porting. The internal chamber volume is ~.55ci. The pressure behind the ball never goes above 60 - 70 psi due to the powertube restriction, even though the available pressure is as high as 550psi. Another factor, is that the volume is getting larger behind the ball but the amount of available air molecules remains constant. Thus, the pressure gets reduced as the ball starts to move away from the bolt, but at a factor greater than the change in volume, due to the powertube restriction.
Basically, when you do your calculations, take into account the restrictions in air flow from the air chamber to the barrel. The real pressure used for the calculations of acceleration are the values found at the end of the bolt, not the pressure found in the chamber of the gun.
Basically, when you do your calculations, take into account the restrictions in air flow from the air chamber to the barrel. The real pressure used for the calculations of acceleration are the values found at the end of the bolt, not the pressure found in the chamber of the gun.
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