basic bolt and barrel motions and the related forces. No attempt will be made to discuss the straightforward machine design methods by which the results are applied in arriving at the particular physical form of the mcchanisms. Also, no detailed computations are made to cover the cffccts of such factors as friction or the incidental forces imposed on the breech mechanism by the auxiliary mechanisms such as the feeder, firing device, or locking device. These effects will have only a relatively slight influence on the bolt and barrel motions. In any ease, they can be properly taken into account only in the advanced stages of a design when the form of the gun mechanism becomes fairly well established. At this point, the preliminary analysis of the bolt and barrel motion can easily be modified as desired.

The analysis which follows is based on the assumption that a particular cartridge with known characteristics is to be used and that the desired muzzle velocity and barrel length have been predetermined.

It is also assumed that all necessarv interior ballistics

data are known and that graphs showing the time variation of pro jectile velocity and chambcr pressure are available (figs. 2-4, 2-5, and 2-6).

NOTE : For some design problems, all or part of this information may not be available. Analytical methods by which the required data and graphs can be approximated for use in preliminary studies mav be determined bv con-• / /

ventional interior ballistics computations. In the preceding description of the factors involved in short recoil operation, considerable emphasis was placed on the importance of keeping the weight of the recoiling parts to a minimum in order to achieve a high rate of fire. As has been mentioned previously in this publication, the weight of the recoiling parts of any gun will be affected not only by requirements for strength, rigidity, and durability but will also depend to a large extent on the particular configuration selected by the designer. (To repeal the example previously cited, if the designer wishes to have the bolt slide in a long barrel extension, the recoiling parts might be heavier than thev would be if the bolt moved on guide rails which were part of the receiver.) Therefore, the weight of the recoiling parts can not be determined with any accuracy until the barrel has been designed and the remainder of the mechanism has been laid out at least to the extent which will make it possible to make a fair preliminary estimate of what weights will he involved. In the proccss of planning the mechanism, it will also be ncccs y ^

determine what distances the parts must travel.

Of coursc, the final dimensions and weights of some of the recoiling parts can not be defined until complete consideration is given to the forces which

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