The pressure behind the
projectile varies from twelve to fifteen tons per square inch, but it
is liable to rise to seventeen and eighteen tons, and in the present
state of the manufacture of gunpowder we cannot in ordinary guns
regulate it nearer than that. It is not a matter of so much importance
so far as the guns are concerned, when using ordinary projectiles, as
the gun will endure a pressure of from twenty-five to thirty tons per
square inch; but with high explosives in the shell it is a vitally
serious matter. From all I can learn regarding European practice, it
appears that not only are the explosives made sluggish, but the
quantity seldom exceeds thirty per cent. of the weight of the shell,
and the velocities, notwithstanding, are kept very low. In the
pneumatic gun the velocity is low also, but so is the pressure in the
gun. The pressure in the firing reservoir is kept at the relatively
low figure of 1,000 pounds per square inch or less, and the air is
admitted to the chamber of the gun by a balance valve which cuts off
just the quantity of air (within a very few pounds) that is required
to make the shot. The gun is long, and advantage is taken of the
expansion of the air.
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