The series of experiments we are about to describe has recently been
made by Mr. Horatio Phillips, a practical gun maker of London. The
results will no doubt prove of interest to those concerned in the use
or manufacture of firearms.
The reason that the two barrels of a shot gun or rifle will, if put
together parallel, throw their charges in diverging lines has never
yet been satisfactorily accounted for, although many plausible and
ingenious theories have been advanced for the purpose. The natural
supposition would be that this divergence resulted from the axes of
the barrels not being in the same vertical plane as the center line of
the stock. That this is not the true explanation of the fact, the
following experiment would tend to prove.
[Illustration: EXPERIMENTS WITH DOUBLE-BARRELLED GUNS.]
Fig. 1 represents a single barrel fitted with sights and firmly
attached to a heavy block of beech. This was placed on an ordinary
rifle rest, being fastened thereto by a pin at the corner, A, the
block and barrel being free to revolve upon the pin as a center.
Several shots were fired both with the pin in position and with it
removed, the barrel being carefully pointed at the target each time.
No practical difference in the accuracy of fire was discernible under
either condition. When the pin was holding the corner of the block,
the recoil caused the barrel to move from right to left in a circular
path; but when the pin was removed, so that the block was not attached
to the rest in any way, the recoil took place in a line with the axis
of the bore. It will be observed that the conditions which are present
when a double barreled gun is fired in the ordinary way from the
shoulder were in some respects much exaggerated in the apparatus, for
the pin was a distance of 3 in. laterally from the axis of the barrel,
whereas the center of resistance of the stock of a gun against the
shoulder would ordinarily be about one-sixth of this distance from the
axis of the barrel. This experiment would apparently tend to prove
that the recoil does not appreciably affect the path of the
projectile, as it would seem that the latter must clear the muzzle
before any considerable movement of the barrel takes place.
With a view to obtain a further confirmation of the result of this
experiment, it was repeated in a different form by a number of shots
being fired from a "cross-eyed" rifle,[1] in which the sights were
fixed in the center of the rib. Very accurate shooting was obtained
with this arm.
[Footnote 1: A cross-eyed rifle is one made with a crooked stock
for the purpose of shooting from the right shoulder, aim being
taken with the left eye.]
A second theory, often broached, in order to account for the
divergence of the charge, is that the barrel which is not being fired,
by its _vis inertia_ in some way causes the shot to diverge. In order
to test this, Mr. Phillips took a single rifle and secured it near the
muzzle to a heavy block of metal, when the accuracy of the shooting
was in no way impaired.
So far the experiments were of a negative character, and the next step
was made with a view to discover the actual cause of the divergence
referred to. A single barrel was now taken, to which a template was
fitted, in order to record its exact length. The barrel was then
subjected to a heavy internal hydrostatic pressure. Under this
treatment it expanded circumferentially and at the same time was
reduced in length. This, it was considered, gave a clew to the
solution of the problem. A pair of barrels was now taken and a
template fitted accurately to the side of the right-hand one. As the
template fitted the barrel when the latter was not subject to internal
pressure, upon such pressure being applied any alterations that might
ensue in the length or contour of the barrel could be duly noted. The
right-hand barrel was then subjected to internal hydrostatic pressure.
The result is shown in an exaggerated form in Fig. 2. It will be seen
that both barrels are bent into an arched form. This would be caused
by the barrel under pressure becoming extended circumferentially, and
thereby reduced in length, because the metal that is required to
supply the increased circumference is taken to some extent from the
length, although the substance of metal in the walls of the barrel by
its expansion contributes also to the increased diameter. A simple
illustration of this effect is supplied by subjecting an India-rubber
tube to internal pressure. Supposing the material to be sufficiently
elastic and the pressure strong enough, the tube would ultimately
assume a spherical form. It is a well known fact that heavy barrels
with light charges give less divergence than light barrels with heavy
charges.
After the above experiments it was hoped that, if a pair of barrels
were put together parallel and soldered only for a space of 3 in. at
the breech end, and were then coupled by two encircling rings joined
together as in Fig. 4, the left-hand ring only being soldered to the
barrel, very accurate shooting would be obtained. For, it was argued,
that by these means the barrel under fire would be able to contract
without affecting or being affected by the other barrel; that on the
right-hand, it will be seen by the illustration, was the one to slide
in its ring.
A pair of able 0.500 bore express rifle barrels were accordingly
fitted in this way. Fig. 3 shows the arrangement with the rings in
position. Upon firing these barrels with ordinary express charges it
was found that the lines of fire from each barrel respectively crossed
each other, the bullet from the right-hand barrel striking the target
10 in. to the left of the bull's eye, while the left barrel placed its
projectile a similar distance in the opposite direction; or, as would
be technically said, the barrels crossed 20 in. at 100 yards, the
latter distance being the range at which the experiment was made.
These last results have been accounted for in the following manner:
The two barrels were rigidly joined for a space of 3 in., and for that
distance they would behave in a manner similar to that illustrated in
Fig. 2, and were they not coupled at the muzzles by the connecting
rings they would shoot very wide, the charges taking diverging
courses. When the connecting rings are fitted on, the barrel not being
fired will remain practically straight, and, as it is coupled to the
barrel being fired by the rings, the muzzle of the latter will be
restrained from pointing outward.
The result will be as shown in an exaggerated manner by the dotted
lines on the right barrel in Fig. 3.
It would appear from these experiments that when very accurate
shooting is required at long ranges with double-barreled rifles, they
should be mounted in a manner similar to that adopted in the
manufacture of the Nordenfelt machine gun, in which weapon the barrels
are fitted into a plate at the extreme breech end, the muzzles
projecting through holes bored to receive them in a metal plate. No
unequal expansion would then take place, and the barrels would be free
to become shorter independently of each other. We give the above
experiments on the authority of their author, who, we believe, has
taken great pains to render them as exhaustive as possible, so far as
they go.--_Engineering._
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