In the
pneumatic gun, the complete control of the pressure remedies the most
important of the mortar's defects and makes the fire accurate from
long ranges down to within a few yards of the gun. It is obvious that
the pressure can be usefully controlled in two ways: (1) by keeping
the elevation of the gun fixed and using a valve that can be set to
cut off any quantity of air, according to the range desired; (2) by
keeping the pressure in the reservoir constant, and using a valve
which will cut off the same quantity of air every time, changing the
elevation of the gun according to the distance. Another important
discovery consists in the application of subcalibered projectiles for
obtaining increased range.
The gun is smooth-bored and a full-sized projectile is a cylinder with
hemispherical ends, to the rear of which is attached a shaft having
metal vanes placed at an angle, which causes the projectile to revolve
round its longer axis during flight. A subcalibered projectile,
however, being of less diameter than the bore of the gun, has the
vanes on its exterior, and is held in the axis of the gun by means of
gas checks which drop off as the projectile leaves the muzzle.
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