The Machine Gun

The barrel pressure curve is reproduced in fig. 3-5. The other curve shown in this figure is the pressure in the gas cylinder. This curve was drawn under the assumption that the amount of gas which flows into the cylinder is too small to have any appreciable effect on the pressure in the barrel and also that the orifice is fairly large so that the pressure in the cylinder builds up rapidly to a value which is practically equal to the barrel pressure. Since the barrel pressure decreases smoothly, the pressure in the cylinder will remain very close to the barrel pressure. If the gas port remains open after the projectile has passed, the pressure in the gas cylinder will become zero at essentially the same time the residual barrel pressure becomes zero. (Ordinarily, however, the orifice is relatively small and produces a throttling effect which causes the pressure in the cylinder to remain considerably lower

/ v than the barrel pressure for most of the time of action.)

Examination of fig. 3-5 will show that the gas piston is subjected to a driving force for only a very short interval of time. The pressure starts to act on the piston when the projectile passes the port at 0.00166 second and ceases to act at about 0.008 second when the residual barrel pressure has reached zero. Therefore the total time of action is 0.00800—0.00166=0.00034 second. For the assumed conditions, it is during this brief interval that the piston absorbs the impulse imparted to it by the powder gases. Since the powder gas pressure exists for such a short time, it is not practical to at-

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