This process of gradually increasing the charges, loading and firing a scries of shots and plotting the pressure and velocities obtained, continues until the maximum permissible pressure for the cartridge is reached, or the pressures become erratic. Rifles can be mounted as pressure gages for routine tests but for experimental work with powders of unknown characteristics, special heavy guns are used that will withstand tremendous pressures.
The result of this firing leaves a sheet of cross section paper with two scries of dots on it; one representing pressures in relation to the weights of charges and the other representing velocities, also in relation to the weights of charges. Velocities can be taken quite accurately and the velocity points will represent quite an orderly progression. The ordinary method of taking pressures is somewhat crude, though satisfactory, and points representing pressures, when connected by straight lines, bear more resemblance to a portrayal of lightning or an Indian tepee than they do to a curve, so these "curves" must be smoothed out. This is done by drawing a regularly curved line that will, without breaking its regularity, pass through as many of the dots as possible. (See frontispiece.)
If the loads first fired were below the tolerance of the powder, the dots or points representing the "curves" will vary up and down. If the firing is carried beyond the upper limit of the tolerance, the points will vary up and down, i.
An automatic pistol case which has been fired to destruction with mercuric primer. Originally flred with a mercuric primer, it was then reloaded and fired three times with non-mercuric primers—
with the result shown above.
These cartridges fired prematurely from the slam of the breech block because the primers projected slightly above the case heads. Note the mark of the firing pin hole and the absence of firing pin indentation.
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