Figure 16. Advantage of Using Special Cartridge Base Form for Blowback.

point shown in fig. 1 6A (as is necessary to allow for the presence of the extractor in the extractor groove). If a cartridge case with the spccial base form shown in fig. 1-6B is used, the cartridge may be pushed further into the chamber and when it is blown back it may therefore travel a greater distance before the walls near the base become unsupported. By this means, the allowable bolt velocity during the first. 0.01 second may be increased somewhat but it should be realized that even with this provision, the velocity will still he limited to a relatively low value.

IMPORTANT NOTE: There are two extremely significant points which may be arrived at from further consideration of the foregoing rules:

Fig. 1-7 shows graphically the manner in which the bolt velocity varies in a plain blow-back gun during the first 0.010 second of the cycle of operation. The shape of the curve-shows that the velocity increases rapidly for the first one or two thousandths of a second and then, as the powder gas pressure falls, it increases much more gradually. {The shape of the curve will be the same for any plain blow-back gun. Only the vertical rcalf, will vary, depending on the particular gun design.} If the gun is designed to permit an average bolt velocity of one foot per second during the first 0.0015 second in order to comply with the first rule, the scale of the graph is then determined so that the average velocity over 0.010 second will be approximately two feet, per second, which is the limit required in accordance with the second rule.

In other words, if lubrication is employed to prevent ease separation in an attempt thus to escape the limitation of the first rule, the bolt velocity still can not be increased because this will result in exceeding the limitation imposed by the second rule. This means that to gain any significant benefit from lubrication in a plain blowback gun, special steps must be taken to permit a greater average bolt velocity than two feet per second during the first 0.010 second. (As pointed out previously, some moderate improvement in this direction can be achieved by the use of the special type of cartridge case shown in fig. 1 6B.)

The important point involved in these considerations is that, without lubrication, the results of using both of the given rules are so similar that either rule may control the design of the gun. If lubrication is used, the second rule will control, and the average bolt velocity allowable in the first 0.010 second will still be two feet per second unless some spccial means are employed to increase this limit. In any case, the average bolt velocity must be limited to a verv small value, somewhere in the order of

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