Room

of the bolt when the bolt is blown back and then drives the bolt back to the firing position).

Cycle of Operation

The automatic cycles of operation for guns employing simple blowback are all more or less the same and occur as follows:

The cycle starts when the bolt is driven against the base of the cartridge in the chamber, firing the round. When the cartridge is fired, the pressure of the powder gases drives the projectile through the barrel of the gun and at the same time immediately drives the cartridge ease to the rear against the resistance offered by the bolt. At this point, the only significant resistance to the acceleration of the bolt is due to the inertia of the bolt mass.

The force exerted by the powder gases exists for a relatively very short time. In a typical 20-inrn gun with a total barrel length of slightly over five feet, the projectile leaves the muzzle after approximately 0.0023 second and the residual gas pressure continues to act for another 0.008 or 0.009 second. Thus the bolt is subjected to accelerating forces only during the first 0.008 or 0.009 sccond after ignition of the primer. After this point, the powder gas pressure is zero and the force driving the bolt back is also zero but the case and bolt continue to move of their own momentum. As the bolt moves back, the case is extracted from the chambcr and cjected. The bolt slows down as its motion compresses the driving spring until the resistance of the spring combined with the action of the buffer has decreased the bolt velocity to zero. At this point, all of the kinetic energy originally imparted to the bolt (except for losses resulting from extraction, ejection, and friction) is stored in the spring as potential energy.

The driving spring then pushes the bolt forward. As the bolt moves forward, it cocks the firing mechanism, picks up a fresh cartridge from the feed mechanism, and carries this cartridge into the chamber. Just before the bolt reaches its fully forward position, the potential energy stored in the driving spring has been transformed back into kinctic energy of the bolt and cartridge (except for losses occasioned in feeding the cartridge, in cocking the firing mechanism, and in overcoming friction). Therefore the bolt is moving with considerable velocity and the kinetic energy resulting from this velocity is absorbed by impact at the end of the forward travel. As the bolt comes to rest, ignition occurs and a new cycle begins.

Analysis of Plain Blowback

In the preceding description of blowback operation, ii was pointed out that the most critical factor affecting the design of a gtin employing this system is the movement of the cartridge case during the action of the powder gas pressure. There arc two considerations relating to this movement which impose definite limitations in the design.

1. If no lubrication is provided, the high pressures generated in the early part of the explosion will cause the cartridge case to seize in the chamber. Therefore, separation of the case will result unless the movement of the bolt is limited so that the allowable elongation of the ease material is not exceeded while the case is stuck. Although the precise elongation limit for a specific cartridge case can be determined only by careful experimentation, a good rule for the brass eases of 20-mm rounds is that the bolt movement should not exceed 0.015 inch during the first 0.0015 second of the propellant explosion. In other words, the average velocity of the bolt during this time should not exceed 10 inches per sccond or at the most one foot per sccond.

2. Even if chamber seizure can be avoided by means of adequate lubrication, the cartridge case can not he permitted to move out of the chamber so far that its thin walls do not receive any radial support while the residual pressure is still fairly high. If this were permitted to happen, it could easily result in swelling or bursting of the case near the base. Here again, the exact limit for a particular cartridge case can only be determined experimentally, but a good rule to follow for brass 20-mm cases is that the movement should not exceed 0.250 inch during the first 0.010 second of the propellant explosion. By this time, the residual pressure will be zero or at least so low that further movement of the case can occur without any danger. This means that during this time the average velocity of the bolt should not cxcccd 25 inches per sccond or approximately two feet per sccond.

This limit is based on the use of an ordinary cartridge case which enters the chambcr only to the q250- inch

0 0

Post a comment