258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1914. 
a physical test is prescribed wherein grains are tested by being 
shortened in a press. The requirement in this test is that no crack 
shall be developed in the outer surface of the grain before it has 
.shortened a specified percentage. Samples representing every 
lot of powder, lots usually consisting of 100,000 pounds, are sub- 
jected to all the foregoing tests as well as to others of less im- 
portance. 
ACCURACY LIFE OF GUNS. 
In order to maintain an elongated projectile in accurate flight it 
must be given a rapid rotation about its longitudinal axis. Except 
for small arms projectiles this is accomplished by means of a copper 
band secured to the projectile and engaging in the raised elements 
or rifling in the gun, this having the desired twist. In small arms 
the rifling engages the projectile over the entire length of the body or 
cylindrical part. It is found that the large powder charges and 
high pressures used in modern cannon rather quickly wear away the 
raised elements, especially near the origin, and that eventually the 
projectile bands fail to properly engage the rifling and the gun loses 
its accuracy. 
With the largest caliber guns this occurs after the firing of from 150 
to 250 rounds, depending upon the weights of powder charges and 
the muzzle velocities used, a large charge and a high muzzle velocity 
in general greatly shortening the accuracy life. For smaller guns the 
accuracy life is greater, amounting to at least 3,000 rounds for a 
3-inch field gun. After a gun has lost its accuracy, it can be restored 
to good condition at a moderate cost by boring it out and adding a 
new lining tube, then reriflmgthe gun. One of the great advantages in 
the use of nitrocellulose powder, that used in this country, in compari- 
son with the nitroglycerin powder cordite, used to a considerable extent 
abroad, is the much less erosive effect of the former. The great 
difference is evident from the fact that in changing from nitroglycerin 
powder to nitrocellulose powder in the United States service small 
arms some years ago, the accuracy life was raised from about 3,000 
to about 15,000 rounds. Plate 3 indicates the condition of the in- 
terior of a 0.30-caliber machine-gun barrel after a rapid-fire test of 
3,000 rounds in which nitroglycerin powder was used. The firings 
were made with unusual rapidity and without the presence of the 
usual water jacket surrounding the barrel, the test being one of a 
series in an effort to find a steel more resistant to erosion than the 
varieties now in use. The water-filled jacket commonly used in 
service firings tends to keep down the temperature of the barrel and 
thus reduces the erosion. 
