7. Impact of piston on bolt
When the cycle of operation has progressed for 0.005 second, as described up to this point, the residual pressure has decreased to the assumed safe operating limit of 750 pounds per square inch and it is now possible to increase the velocity of the bolt without danger of rupture of the cartridge case. It should be noted that, at 0-005 second, the residual powder gas pressure has not yet reached zero and therefore some blowback action will occur during and after the impact of the piston.
The first point to consider is what bolt velocity is desired after impact in order to obtain the required rate of fire. For the specified rate of fire of 1200 rounds per minute, the time available for the bolt to complete its rearward travel is about 0.022 second (slightly less than half the cycle time). The required bolt travel with respect to the gun to provide an opening sufficient for feeding will be taken as 10 inches (0.833 foot). Since the gun recoils 1.5 inches or 0.125 foot, the total bolt travel is .833+. 125—.958 foot. At 0.005 second the bolt has already moved 0.0507 foot and therefore the remaining travel of 0.907 foot must be accomplished in 0.017 second. This means that the bolt must move this distance at an average velocity of:
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