For Mm

Vp-2750FT/SEC. Wp-0.290 LB. Wc»0.070 LB. BORE TRAVEL«4.3FT.

Figure 3-17. Graph of Chamber Pressure Versus Time (20 mm Gun).

liminary studies may be determined by conventional interior ballistics computations.

As the analysis progresses, its applications will be illustrated by means of sample calculations. Although these calculations and the related graphs are for a specific 20-mm cartridge and barrel and are based on certain assumed weights and other characteristics, the general approach described is applicable to gas operated guns of any caliber. The calculations cover the following important points:

1. Determination of the conditions of free recoil.

2. Determination of the correct time for unlocking.

3. Computation of data required for design of the gas actuating device.

4. Determination of data required for timing the operation of the piston.

5. Selection of characteristics of barrel springs and bolt driving spring and determination of data for backplate buffer and buffers associated with barrel spring. 6. Development of graphs show how the velocity and travel of the gun, piston, and bolt vary with respect to time.

In the course of describing these calculations, the following fundamental formulas will be developed and explained.

a. Momentum and velocity relations for time projectile is in bore b. Formula for determining velocity of free recoil c. Expression for duration of residual pressure d. Formulas for determining spring retardations. {Before proceeding, it should be mentioned that the action of a gas-operated gun up to the time the piston strikes the bolt Ls very similar from the analytical point of view to the operation of a short-

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