the field artillery journal - Fort Sill - U.S. Army

SUPPLEMENTAL NOTES ON LOST MOTION
the projectile in the bore; this extreme condition was
emphasized in the words, "even under the extreme assumption
of maximum powder pressure acting during the entire period."
The test with subcalibre ammunition gave one extreme where
no "jump" of the gun occurred, and by way of comparison the
theoretical discussion treated the other extreme, the gun in free
motion actuated to the time the projectile cleared the muzzle by
the maximum pressure given in the powder specifications,
33,000 lbs. per square inch. The futility of a mathematical
treatise of the hair-splitting variety on the subject of jump was
well appreciated, and owing to the fact that no authentic firing
data were available at the time the original computations were
made, a precise calculation of the jump to be expected under
normal conditions was not attempted. The trend of thought
expressed mathematically was offered only as a guide or
compass to indicate the proper direction when regulated by
common sense and experience. Powder pressures are not static
but dynamic, in which we deal with the duration of the greatest
measured pressure or a period of time infinitesimal. The
element of time changes the effect of a given force, producing
in the case of a given powder pressure less strain in the metal
and consequently less displacement of the mass of metal than
would result from a static pressure as hydraulic pressure which
is comparatively much longer sustained. The case is analogous
to that of the small arms rifle where the hydraulic tests of
barrels in past records show an elastic limit of about 40,000 lbs.
per square inch interior pressure. Nevertheless, the same barrel
has been fired as many as 100 rounds with excessive pressure
charges of 100,000 lbs. per square inch without bursting. Surely
the average effective pressure down the bore for the entire
period that the projectile is in flight in the 3-inch gun is far
less than the maximum and instantaneous pressure recorded
by our gauges, some 30,000 lbs. per square inch. Were we to
imagine the average effective pressure producing acceleration of
the gun to be 33,000 lbs. per square inch, and the gun free to
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