Cylinder Cammed Forward To Form Seal

camming mechanism. On the other hand, a seal of the form shown in fig. 4-/B will tend to be expanded by the explosion of the propcllant charge. Therefore the mating surfaces are forced together, not only by the thrust of the camming mechanism, but also by the extremely high pressure of the powder gases. Under the combined actions of these forces, a very tight seal is effected and leakage is practically nonexistent. It is interesting to note that this excellent sealing method can be found in percussion cap revolvers which date from more than 100 years ago.

Although good scaling can be obtained in a revolver by camming the cylinder forward and using sealing rings, the method is accompanied by certain difficulties. The necessity for the sliding motion of

« o the cylinder complicates the cylinder mounting and increases the problems involved in the design of an accurate cylinder indexing mcchanism. Also, the reciprocating motion which must be imparted to the cylinder requires additional mcchanism to be built into the gun and this mcchanism must be ruggedly constructed in order to handle the large forces it must absorb. Furthermore, the forward motion of the cylinder must occur before a round is fired and then, after the chamber pressure has decreased to a safe value, the cylinder must be withdrawn before it can be rotated to place the next round in the firing position. Because the cylinder is of necessity a fairly heavy part and therefore has considerable inertia, both the forward and rearward motions will require some small but significant amount of time which will cut into the time available for the other events of the firing cycle. In a high-rate-of-fire gun where the firing cycle time is in the order of only a few hundredths of a sccond, the time required for moving the cylinder in and out can easily have a tendency to slow down the rate of fire. Another important point is that the sealing rings must be quite soft to function properly and hence can be dented or deformed easily. If such damage to a ring occurs either through some malfunction of the mechanism or as the result of careless handling during cleaning or maintenance, it is likely that the high-pressure powder gases will quickly blow the seal out entirely, thus producing a very dangerous condition.

The next sealing method for consideration is that in which the cartridge case is pushfcd forward so that the neck of the case enters the barrel for a short distance. Here, again there are several possible ways of arranging the mcchanism. In one method, as shown in fig. 4 8, a so-called "buried" projectile is used, that is, the neck of the cartridge case extends entirely over the projectile. When the cartridge is seated in the chamber opening, a small portion of the neck of the case projects beyond the front face of the cylinder. After the chamber has been positioned behind the barrel, the entire cylinder is moved forward to closc the gap between the cylinder and barrel and the projecting neck of the cartridge case enters the barrel. The neck of the case thus spans the small scam where the cylinder joins the barrel. The explosion of the propellant charge causes the cartridge case material to be expanded tightly against the inside of the barrel so that the gas pressure

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