The Machine Gun

transmitted through the piston to the barrel and thence to the frame while the rearward force is also transmitted to the frame through the cylinder and cartridge case. These forces arc equal and opposite and therefore do not intensify the recoil but they do act to cause an additional tension in the frame. As previously shown, this additional force can be almost 98,000 pounds (for a 30-mm gun with a peak chamber pressure of about 65,000 pounds per square inch). Since the effective force on the base of the cartridge case due to the pressure acting on the bore cross-section area is only approximately 74,000 pounds, it is evident that the use of the piston seal can more than double the tension in the frame. With a total tension of 172,000 pounds instead of only 74,000 pounds, it will be necessary to use a in order to withstand the forces to which they are subjected, piston seals must have a substantial wall thickness. If an attempt is made to counterbore the chambers for receiving suitable seals, a situation like that shown in fig. 4-12B may result. The countcrborcs might actually overlap or the walls remaining between them might be entirely too thin for safety. To provide sufficient room for the coun-terbores, it is necessary to respace the chambers as shown in fig. 4-12C. Therefore, the cylinder will be larger and heavier and the design will suffer from all of the consequent disadvantages. Of course, these remarks apply only when the ammunition is nearly cylindrical in form, that is, when the cartridge case is not much larger in diameter than the projectile. WThen the cartridge case is strongly

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