Pressure Producing Motion

Figure 1-3. Effects of Axial Pressure.

is free to slide in the chamber, this net force is equal to the chamber pressure multiplied by the total cross-section area of the mouth of the case (not the projected area of the inside of the base}. (See fig. 1—3.) If the projected area of the base of the cartridge ease is greater than the area of the mouth, the forcc on the base is of course greater than the net force. However, the difference between the forces is cancelled by Ihe axial component on the tapered or necked portion of the case. (This component is directed forward.} Note that if the entire case were free to move, the difference between the net force and the force (in Ihe base would cause a tension in the walls of the case.

If the bolt were rigidly locked and held firmly against the base of the cartridge case (that is, if there were ?ero excess head space), the gas pressure would merely compress the material of the cartridge ease against the chamber walls and bolt face. However, with blowback operation» the ease and the bolt, must be free to move at some predetermined time during the action of (he gas pressure so that the required energy can be transmitted to the bolt. For purposes of examining the effects of this movement, it will be assumed that the bolt is not locked at any time and that it resists movement onlv hy its inertia. (As will be pointed out later, these conditions do not apply for all blowback weapons.) The movement, of Ihe cartridge case can be considered to occur in the following three phases: Phase 1. The pressures which exist during the first 0.0001 second of the propellant explosion arc relatively low hut. are sufficient to expand the thin brass near the mouth of the cartridge case against the chambcr wall, thus forming the. seal which prevents the powder gas from escaping to the rear. Sincc the radial pressure is not extremely high during this phase, the friction between the cartridge case and the chamber is not excessive. Therefore, the axial component of the pressure causes the entire case to stretch or slide back slightly in the chamber so that it first takes up any excess head space which may exist and then starts to impart motion to the bolt. Bccausc of the high inertia created by the mass of the bolt.

the velocity of this motion is relatively low.

NOTE: Jf the excess head space is very large, it may not be taken up entirely before tlie cliftll) ber pressure builds up to a high value. In this event, the conditions described in Phase 2 are applicable.

Phase 2. The sccond phase of the cartridge case movement occurs during the period of extremely-high pressure on either side of the peak shown in fig. 1 1. The behavior of the cartridge rase during this phase depends entirely upon whether or not the case is suitablv lubricated.

If the cartridge case is entirely unlubricatcd or is

Insufficiently lubricatcd, it is expanded heavily against the chamber walls, producing a high-pressure metal-to-metal contact which results in a vcrv

high friction between the ease and the chambcr. Because of the friction between the walls of the case, and the chambcr, the pressure acting against the. base of the case will result in a tensile stress in the walls of the case. In fact, the forward portion of the ease may he gripped so tightly during the period of high chamber pressure that the fractional resistance excccds the tensile yield strength of the case walls. In this event, the forward portion of the case sticks to the chamber wall but the rear portion continues to move, causing the case to stretch plastically. If the bolt does not provide sufficient resistance to prevent the strctching of the case from exceeding the. allowable elongation of the ease male-rial (about 0.015 inch), the ease will separate.

At this point, special mention should be made of what effcct excessive head space would have if an attempt were made to use unlubricated ammunition. As pointed out in Phase 1, the motion of the case may not take up all the head space before the chamber pressure builds up to a value high enough to cause the forward portion of the cartridge case, to stick. Assuming that some excess head space still remains, the base of the case will be unsupported and the high pressure will cause the case to be stretchcd. If the excess head spacc is so large that it is not all taken up before the stretch excceds the allowable elongation of the case material, separation will occur.

NOTE: It should be realized that the forccs produced by the peak chamber pressures are vastly in excess of the strength of the thin brass of the cartridge case. With forces of this magnitude. the case strctches quite easily. This may be illustrated as follows: With the forward portion of the case stuck, the force tending to stretch the case Is equal to the pressure multiplied by (he area of the inside of the base. Since the maximum chambcr pressure for a 20-mm cartridge is 45,000 psi and the inside base area is approximately 0.5 square inch, the stretching forcc is in the neighborhood of 22,-000 pounds. The area of metal in tension for the case wall may be approximately 0.1 square inch and the ultimate strength of the half-hard brass of which the ease is made is about 50,000 psi. Therefore, the maximum resistance which the case can offer to stretching and separation is only approximately 5000 pounds. It easily can be seen that this resistance is practically negligible when compared to the stretching force of 22,000 pounds. In fact, forccs sufficient to produce separation can occur whenever the chamber pressure is over 10,000 psi, and as shown in fig. 1 1, such pressures exist for almost the entire time the projectile is still in the bore. (Note that the force expended in overcoming friction or in stretching the case rcduces the forcc applied to the bolt. If this effect is excessive, the gun may-fail to continue firing because the. energy transmitted to the bolt is insufficient to operate the gun mechanisms).

The preceding description applies to the condition in which the case is either entirely unlubricated or insufficiently lubricatcd. If a fairly thick film of a suitable lubricant is applied between the cartridge case and the chambcr wall, an entirely different condition results. It should not be thought that the purpose of the lubricant is merely to make the cartridge case "slippery". Its true purpose is to form a continuous film which remains between the case and the chambcr wall and effectively "in-sulates>! their surfaces from metal-to-metal contact, even under extremely high chamber pressures. Since there is no metal-to-metal contact to cause seizing and sticking, the ease slides freely in the.

chamber and the onlv resistance to its movement

(except for the resistance offered by the bolt) is the forcc requir ed to produce shear in the lubricant film.

According to established laws regarding friction, the frictional resistance for well-lubricated, smooth metallic surfaces is relatively very low and is practically independent of the pressure between the surfaces. The major considerations are the area of the lubricant film in shear, the viscosity of the lubricant, and the speed of the relative movement between the surfaces. Thus, a well-lubricated cartridge case will move almost as freely under high chamber pressure as under low pressure and therefore proper lubrication should completely eliminate seizing of the cartridge case, which is the basic cause of ease separation.

Although proper lubrication will eliminate seizing of the cartridge case, other difficulties can occur if the ease is permitted to move too far while the chamber pressure Ls extremely high. Excessive movement will result in a condition where the rear portion of the case is out of chamber and will therefore receive no radial support from the chamber walls. The high internal pressure may then cause the case to swell at the base portion or even to rupture. Furthermore, if in the chamber, the seal between the case and chamber mav be broken

thus permitting the escape of hot powder gases at the brccch. (In most rapid-firing blowback guns some escape of gases at the breech is inevitable, but if this effect is excessive it can cause damage to the brccch mechanism or may even be dangerous for operating personnel.)

It is of interest to note a spccial difficulty which can occur with ammunition having a cartridge ease of large diameter when compared to the project ile diameter (bottle-necked or strongly tapered case). With this type of case, the internal pressure tends to produce high tensile stresses in the case walls. (Sec fig. 1-3.) If the case is but tie-necked, the rearward movement of the case crcatcs a space between the shoulder of the case and the chamber. Since this leaves the forward portion of the case unsupported, the internal pressure lends to deform the case by pushing the shoulder forward to fill the spare created by the movement. If the case is of the tapered type, the rearward movement tends to ercatc a gap between the wall of the case and the chamber. Sincc this leaves the wall of the case unsupported, the internal pressure expands the case to close the gap.

Under cither of the conditions described above, the deformation of the case can be of considerable magnitude if the movement of the bolt is cxccssive and it is quite possible that the deformation will exceed the allowable limit of the brass. This may-cause the ease to tear or rupture in such a way as to cause a separation or to cause difficulties in extraction or ejection. For this reason, plain cylindrical cases or cases with only a slight taper are to be preferred for use in blowback guns.

Phask 3. The final phase of the cartridge case movement starts when the chamber pressure has decreased to a level which permits the case to contract, thus reducing the friction to a negligible value. After this point, the remaining pressure continues lo drive the ease to the rear but the. forces on the case arc low enough so that there is no danger of the case failing by separation or rupture. This phase ends as the gas pressure approaches zero (0.008 or 0.009 second after ignition of the primer). Although at this point the driving forcc is rcduced to zero, the case and bolt continue to move of their own momentum with sufficient kinetic energy to complete the extraction and ejection of the case and to operate the gun mcchanisms for the remainder of the automatic cyclc.

Conclusions

All of the points discussed in the preceding description of the behavior of the cartridge case in a blowback gun may be summarized in one very important principle which controls all of the characteristics and fundamental design requirements for this type of weapon. This principle may be stated simply as follows:

THE PRIMARY DIFFICULTIES IN BLOWBACK OPERATION ARE THE DIRECT RESULT OF EXCESSIVE CARTRIDGE CASE MOVEMENT DURING THE PERIOD OF EXTREMELY HIGH CHAMBER PRESSURE AND THESE DIFFICULTIES ARE AGGRAVATED BY INADEQUATE CASE LUBRICATION. In the design of blowback guns, each factor mentioned in this statement must be considered carefully in order to avoid operational difficulties. This analysis leads to the following general conclusions.

Chamber Pressure. All of the difficulties that arise are the direct result of extremely high chamber pressure. Therefore, it follows that if the chamber pressure could be kept low, these difficulties would disappear immediately. Unfortunately, high chamber pressures are essential in high-powered heavy machine guns (which arc the main concern of this publication) and accordingly, the use of low chamber pressure cannot be considered as a solution to the design problem. (The relative case with which the blowback principle can be applied to low-powered small-caliber ammunition is amply demonstrated by the large number of self-loading pistols, sub-machine guns, and light machine guns that have used this principle successfully.)

Cast7- Movement. The major problem confronting the designer of a high-powered blowback gun is how to limit the movement of the cartridge case during the action of the extremely high pressures resulting from the explosion of the propellant charge. This problem and the design difficulties related to it are the dircct result of a number of conflicting requirements.

Since, in blowback weapons, the source of the power for operating the mechanism is derived from the thrust applied to the cartridge case by the powder gases, it is essential for the cartridge case to move while the gas pressure is acting. However, it is this very motion under pressure which causes separations and other damage to the cartridge case. When steps are taken to reduce these difficulties by limiting the movement of the cartridge ease, care must be exerciscd to insure that sufficient energy will be available to operate the. gun efTect.ive.ly at the desired rate of fire. Furthermore, if adequate lubrication of the cartridge ease is provided so that a fairly high bolt velocity can be tolerated during the period of high chamber pressure, it may be found that the resulting rapid extraction of the ease will cause rupture of the case or cause the breech seal to be broken too soon.

The foregoing considerations and other points which will be discussed later cause the design of a blowback gun to be a problem in balancing various critical factors, one against the other, in order to obtain the required performance characteristics. There are. many possible solutions to this design problem and the particular solution employed is what determines the basic features of the gun. A detailed explanation of howr these solutions are applied in practice is given later under the heading '"Blowback Systems".

Lubrication. The importance of providing suitable lubrication for high-powered ammunition used in blowback guns cannot be emphasized too strongly. Expcricncc has shown that without proper lubrication, difficulties such as ease separation, chamber seizure, loss of bolt recoil energy and poor extraction seem to make it impossible to attain high performance in a blowback gun. In fact, so essential Is lubrication to this type of weapon that the term "oil-omatic" has been suggested as being more suitable than "automatic" for use in referring to blowback machine guns.

It. is very important to realize that under practical operating conditions, factors may be encountered which make it verv difficult to maintain suit-

able lubrication for ammunition. For example, guns and ammunition used in arctic operations or carried by aircraft flying at very high altitudes can be subjected to extremely low temperatures that can cause ordinary lubricants to fail completely. It also should be remembered that greasy substances applied to the ammunition or chamber wall easily pick up sand or other contamination that can cause the lubricant film to be broken and causc the cartridge case to seize in the chamber. This problem is particularly serious for any gun which is to be used in the field. All things considered, one of the first things that the designer of a blowback gun must do is find an ammunition lubricant which is suitable both for its intended purpose and for the operating conditions to which the gun will be subjected. If such a lubricant can not be found, it is safe to say that blowback operation will not be practical and should be abandoned in favor of some other system

of operation in which cartridge case lubrication is not of such critical importance.

Although the term "lubricant" usually brings to mind an oil or grease, there arc many other substances which can qualify (at least from the theoretical point of view ) as ammunition lubricants. This is so because the only really important requirements for an ammunition lubricant are that it must form a continuous insulating film and that the force required to produce shear in the film must be smaller than the force required to produce separation of the cartridge case. It can be seen readily that in order to satisfy these requirements, a substance need not have any of the unctuous or "slippery" character usually associated with lubricants used for minimizing friction.

The fact that it is relatively easy to think of almost any number of substances which could satisfy the theoretical requirements might lead one to believe that it would be a small problem to find an ammunition lubricant suitable for any and all conditions. The sad truth of the matter is that such a substance has been sought eagerly but unsuccessfully ever sincc 1898. In this quest of over half a century, designers, inventors, research groups, and just plain practical men have suggested an amazing number of materials which seemed to have possibilities. The list includes waxes, graphite mixtures, a variety of liquids, and solid coatings, all of which are so numerous that recording them here would take too much space. However, controlled tests and the hard lessons of practical cxpcriencc have eliminated them all.

Other more radical attempts to avoid case seizure through the use of such devices as chromed, fluted, or stepped chambers have met with a similar lack of success. These suggested materials and remedies have failed because they all amounted to stepping out of one difficulty into another. For example, one coating which seemed otherwise satisfactory was scraped off the ammunition by the feeder in the form of small shavings that soon gummed up the breech mechanism. Other substances produced excessive fouling in the chamber, and so on.

The result of all this experimentation is that today, as in 1B9B, heavy-bodied oil and grease, with all their attendant difficulties, arc still the only substances which have been used to lubricate ammunition with any reasonable degree of success. Either of them will producc a good film which will provide the required insulating effect and under many conditions of operation the difficulties encountered in their use arc not too serious.

It should be remarked here that light oil, such as sewing machine oil, is not a satisfactory lubricant for ammunition. It may be argued that even light oil can form a film and is just as incompressible as heavier oil or grease and therefore should serve as well to provide the required insulation. However, practical experience has shown that the use of light oil is not cffcctive in preventing case separations. Apparently, the difficulty with light oil is that a sufficiently thick film can not be maintained. As shown in fig. 1-4A, the light oil will form a film which covcrs the entire cartridge case, but there are bound to be small irregularities in the cartridge case wall. When the case is expanded in the chamber by the pressure of the powder gases, as shown in fig. 1 4B, the light oil will be caused to flow off the high spots into the low spaces and since the film is so thin, there will not be sufficient oil to over-fill these spaccs. Therefore, all the oil will be forced off the high spots, thus permitting mctal-to-metal contact to occur at these spots. On the other hand, a sufficiently thick film of a heavier bodied lubricant can be maintained so that even though there is some flow into the low spaces, there is still enough lubricant to cover the high spots (figs. 1 AC and 1-4D).

There is another advantage to the use of heavier bodied lubricants. In any gun, the exploding powder gases produce a flame which cscapcs around the cartridge case during the extremely brief instant required to expand the neck of the case against the

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