which occurs at the joint between the forward end of the cylinder and the rear end of the barrel. An-
other serious difficulty that occurs with long bursts at high rates of fire is the tremendous amount of heat absorbed by the cylinder. Also there is a critical requirement for high precision in the cylinder indexing mechanism to insure that the chambers will be accurately aligned with the barrel bore. Finally, the fact that the projectile must have a rather considerable free run before it engages the rifling in the barrel causes an additional source of trouble. These four problems, although they may-appear at first glance to be simple enough, have been proved by experience to be the stumbling blocks which have stood in the wav of rotarv cham-
ber mechanism development throughout the entire history of modern machine guns. Because of the importance of these problems, they will be covered in some detail in the following paragraphs and the devices which have been so far applied for their solution will be analyzed in order to give an indication of what progress has been made and what work yet remains to be done.
First consider the problem of gas leakage. That this difficulty has existed and has been recognized for many centuries is very clear from the historv of
revolver weapons. In the very early days of firearms, when the tools and techniques for constructing accurately fitted mechanisms were not available, all revolving firearms that enjoyed any success were built on the "pepperbox" principle. That is, they employed a number of complete barrels rather than a single barrel with a separate revolving cylinder containing the individual chambers. Although the concept of using a separate cylinder was conceived at a very early date, it was not until the eighteenth century that the art of machining had advanced sufficiently to permit the actual construction of practical revolving cylinder weapons. It is highly significant that almost immediately after these guns appeared, there were many attempts to provide a positive means of sealing the joint between the cylinder and barrel. These attempts have continued, intermittently, from that day on.
Some of the scaling methods used in the hand guns of the past were fairly effective, but none of them have stood the test of time well enough to survive. In the modern revolver hand gun, the only precaution taken to limit leakage is the provision of a close fit between the cylinder face and the rear of the barrel. This docs not bv anv means
eliminate the leakage, even in a well-constructed weapon. In any such gun that has been fired several times, a considerable amount of powder fowling will be evident around the front of the cylinder and when the gun is fired at night, a substantial flash will be observed at the chamber gap. In this connection, it is interesting to note that a silencer, which works very effectively on a single-shot arm, is useless on a conventional revolver because the blast at the chambcr gap produces just as much noise as the muzzle report. Thus it appears that, for hand guns at least, experience has proved it more practical to ignore the problem of leakage than to complicate the weapon with sealing devices. With pistol ammunition, which is all low-powered, (he leakage problem is just not severe enough to warrant the use of remedial gadgets or special and expensive mcchanisms.
Unfortunately, it is not possible to treat the gas leakage problem so lightly in a revolver weapon which employs high-powered ammunition. Leakage effects wrhich are minor in pistol calibers become major problems at the extremely high chamber pressures produced by modern aircraft cannon ammunition. Earlier in this chapter, the problem of leakage was trea ted in general terms under the heading, "Principles of Multiple Chamber Weapons." At this point, it is appropriate to review the facts which are applicable to revolver weapons in particular and to consider in more detail the various devices used to prevent leakage in these weapons.
When a rotating mechanism weapon is fired, the pressure of the powder gases builds up with extreme rapidity and drives the projectile out of the cylinder into the barrel. It happens in most revolvers that the projectile passes into the barrel at practically the same instant that the chamber pressure reaches its peak value. In a modern aircraft cannon, this peak pressure may be in the vicinity of from 50,000 to 60,000 pounds per square inch, and as the projectilc passes into the barrel, this tremendous pressure is suddenly brought to bear on the joint between the barrel and cylinder. If the joint is not tightly scaled, a very undesirable situation exists. Any considerable gap will result in a violent and destructive blast which could easily ruin the gun and anything near it. Even the presence of a small opening can cause serious difficulties because the high-pressure gases are at such an extremely high temperature and pass
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