ELEMENTS OF GUN CONSTRUCTION AND DESIGN
“A gun is a machine by which the force of expanding gas is utilized for
the purpose of propelling a projectile in a definite direction.” It is
essentially a metal tube closed at one end, of sufficient strength to
resist the pressure of the gases caused by the combustion of the powder
charge in the confined space at the closed end of the tube behind
the projectile. The rapid combustion of the powder, which produces a
high temperature, gives rise to a pressure uniformly exerted in all
directions within the confined space. The energy exerted is used in
forcing the projectile from the tube.
TUBES
Due to the effort of the large amount of superheated gas generated,
which tends to expand in all directions, tremendous rending stresses
are set up in the tube. Formerly these stresses were met by a sheer
mass of metal, but, as the size of the projectiles increased and the
necessary pressure to give them muzzle velocity increased, the size
of the guns increased beyond the practical limits of mobility. This
was at first offset by forgings of refined alloyed steels, but even
these failed to keep pace with the increasing pressure desired. The
new condition was met by the introduction of the “built-up” and the
“wire-wrapped” guns. The modern built-up gun is made by assembling one
or more superimposed cylinders around a central tube. The superimposed
cylinders, whose inside dimensions are slightly smaller than the
outside dimensions of those on which they are to be assembled, are
expanded by heat sufficiently to allow them to be assembled over the
tube. The subsequent contraction on cooling causes each of them to
exert a uniform pressure on the cylinder immediately underneath. This
method of assembling is called “shrinkage.” This gives a compression to
the inner tube and a slight tension to the outer one. The compression
is so much additional strength to the tube because it must first be
overcome before the powder gases can exert a tension on the inner tube
fibers. The exact amount of the compression and tension for all parts
of a gun at rest or resisting an explosion is a matter of mathematical
calculation. The built-up construction has been used in practically all
our present day types of field artillery.
THE WIRE-WRAPPED GUN.
Wire-wrapped guns consist of:
(a) An inner steel tube which forms a support on which the wire is
wrapped and in which the rifling grooves are cut.
(b) Layers of wire wrapped upon the tube to increase its resistance
by the application of an exterior pressure as well as to add to the
strength of the structure by their own resistance to extension under
fire.
(c) One or more layers consisting of a steel jacket and hoops placed
over the wire with or without shrinkage. The jacket generally furnishes
longitudinal strength to the guns, and the breech block is screwed into
the jacket, or into a breech bushing, which is screwed into the jacket.
The principal advantages of this type of gun over the built-up is
economy of manufacture and greater facilities for inspection of
materiel in the layers over the tube. The wire wrapping has itself
a large reserve of strength due to the high elastic limits that may
be given it. Two methods are used to wrap the wire: (a) at constant
tension (b) at varying tension so that when the gun is fired with the
prescribed pressure, all layers of wire shall be subjected to the
same tangential stress. The latter method is theoretically better, but
because of the ease of manufacture, together with the large factor of
safety possible, the wire is usually wrapped at a constant pressure.
THE BUILT-UP GUN.
All army guns except small howitzers or mortars are of the built-up or
wire-wrapped type. Built up guns of less than 5” caliber, or howitzers
up to 8” caliber consist of an inner tube and a jacket shrunk onto this
tube. The jacket covers the breech end of the gun and extends forward
to the center of gravity. Built-up guns of larger caliber have one more
layer of hoops in addition to the jacket, one layer of hoops usually
extending to the muzzle.
The bore of the tube forms the powder chamber, the seat for the
projectile and the rifled bore. Rifling consists of a number of helical
grooves cut in the surface of the bore. The soft metal of the rotating
band of the projectile is forced into these grooves causing the
projectile to take up a rotary motion as it passes through the bore.
This is necessary in order to keep the projectile stable in its flight.
TWIST.
By twist of rifling is meant the inclination of one of the grooves to
the element of the bore at any point. Rifling is of two kinds: (a)
Uniform twist, or that in which the twist is constant throughout the
bore, (b) Increasing twist or that in which the twist increases from
the breech towards the muzzle.
The twist of rifling is usually expressed in the number of calibers
length of bore in which it makes one complete turn. The twist actually
required at the muzzle to maintain the stability of the projectile
varies with the kind of projectile and the muzzle velocity. If a
uniform twist be used, the driving force on the rotating band will be
at a maximum when the pressure in the guns is at a maximum—or near
the origin of rifling (seat of the projectile). The increasing twist
serves to reduce the maximum driving force on the band thus lessening
the danger of stripping the band. This is its principal advantage over
the uniform twist, though it also reduces slightly the maximum pressure
in the gun. The principal disadvantage of the increasing twist is the
continued change in form of the grooves pressed in the rotating band,
as the projectile passes through the bore. This results in increased
friction and a higher value for the passive resistance than with a
uniform twist. (Note: greater ranges obtained by cutting grooves in
projectile, principal used on the long range gun by the Germans.) If
the twist increases from zero at the breech uniformly to the muzzle,
the rate of change in the tangent to the groove is constant. A twist in
this form offers less resistance than the uniform twist to the initial
rotation of the projectile. To still further diminish this resistance a
twist that is at first less rapid than the uniformly increasing twist
and later more rapid has been generally adopted for rifled guns.
Formerly in our service the twist was uniform; one turn in 25 calibres
for guns and one turn in 20 calibres for howitzers. All the latest
model army guns, however, have an increasing twist of one turn in 50
calibres at the breech to one turn in 25 calibres at a point from 2
to 4 calibres from the muzzle. In howitzers and mortars the twist is
sometimes one turn in 40 calibres at the breech to one turn in 20
calibres at a point several calibres from the muzzle. Some mortars are
rifled with a uniform twist and some guns have a rifling which begins
with a zero twist. (The 1905 3” gun, 0 to 1 in 25.)
OUTER CYLINDERS.
Outside of the tube is the jacket. It extends to the rear of the tube
a sufficient distance to allow of seating the breech block. In this
manner the longitudinal stress due to the pressure of the powder gases
on the face of the breech block is transmitted to the jacket thus
relieving the metal of the tube from this stress. In all built-up guns
there is some method devised for locking the tube to the jacket so as
to prevent relative movement of these parts.
Considering the gun alone the greatest range is obtained at an angle of
about 43 degrees from that gun which fires the heaviest projectile with
the greatest velocity. The caliber being limited to from 2.95 inch to
3.3 inch, the projectile is limited in weight to from 12 to 18 pounds.
The weight of the gun is limited to between 700 and 1000 pounds and in
length to between 27 and 36 calibers. The longer the gun, the greater
the weight and velocity from the same charge of powder. A pressure of
33,000 pounds per square inch with a corresponding velocity of 1700
f. s. has been found to be as high a pressure and velocity as are
desirable for a reasonable length of life for a field gun, the average
life of which is 10,000 accurate rounds.
Under the French school of artillery, which dominates our service at
present, our bore is 75-mm, the weight of our shell 12 pounds, our
shrapnel 16 pounds, the velocity for the one about 1,750 f. s. and for
the other about 1,680 f. s.
BREECHBLOCKS.
The breechblock appears in four distinct types. Our own service has for
years used the swinging interrupted screw breechblock which in the 1905
model is the equal of any of that type in existence. The swinging block
has serious disadvantages for high angle fire in that it requires an
excessive amount of room to operate and is difficult to load at high
elevations.
The Italians have introduced a new breechblock in one of their recent
guns, consisting of a half cylinder with superimposed spherical face
on its cylindrical surface rotating vertically about a horizontal
axis perpendicular to the axis of the bore. The gun is loaded through
a groove in the breechblock when the latter is in its horizontal
position. The block, which is semi-automatic, is very satisfactory. It
is adapted to high angle fire.
The French in their “75” have used the rotating eccentric screw type,
which is rapid in movement and lends itself fairly well to high angle
fire. It is completely enclosed and of rugged construction.
The Germans have used the sliding wedge type of block, moving in a
horizontal direction, which does not lend itself to high angle fire.
The United States in its recent field carriage adopted the sliding
wedge type in a vertical plane on account of its manifest superiority
in fire at high angles. This block is rather difficult to manufacture
and the type has a tendency to stick. The automatic closing
necessitates a strong closing spring which fatigues the block operator,
No. 1 in the gun squad. It is interesting to note that in a prospective
new design for the 1916 gun carriage the American Ordnance Department
adopted the French breechblock; and the St. Chamond Company, designing
for the American Expeditionary Forces, adopted the American drop block.
Requirements for a breech mechanism:
The following may be said to be the principal requirements for a
successful breech mechanism.
1. _Safety._ To be safe: (a) the gas must be restrained from escaping
to the rear; this sealing or obturation must be automatic, greater
pressure insuring better obturation. (b) The breech of the gun must
not be weakened by the fitting of the breech mechanism. (c) The parts
must have ample strength to prevent any portion from being blown to the
rear. (d) The danger of premature discharge must be minimized. (e) The
breechblock must be securely locked to prevent opening on firing.
2. _Ease and Rapidity of Working._ Otherwise, rapid and continuous fire
cannot be maintained. Hence this would include facility in loading and
certainty of extraction for rapid fire guns.
3. _Not Easily Put Out of Order._ In other words it must be able to
meet service conditions and hard usage. Parts should have a reserve
strength.
4. _Ease of Repair._ Parts most exposed to wear should be so designed
as to permit being replaced. This will also include accessibility to
parts, so that breakage of a part will not disable the mechanism for a
long time.
5. _Interchangeability._ Not only should individual parts be made
interchangeable by accurate workmanship, but the whole mechanism should
be capable of being mounted on similar guns. This is to meet service
conditions.
GUN CARRIAGES.
A modern gun carriage is expected to stand steady on firing, so that
in the first place it requires no running up, and in the second place
it maintains the direction of the gun so that only a slight correction
in elevation and direction is required after each round. The carriage
is maintained in position by the spade, which sinks into the ground,
and by the friction of the wheels upon the ground. If the force of the
recoiling gun were communicated directly to the anchored carriage the
effect would be to make it jump violently, which would not only disturb
the lay, but would prevent the cannoneers from maintaining their
position. The hydraulic recoil brake is therefore interposed between
gun and carriage. If the guns were rigidly attached to the carriage
the latter would be forced back a short distance at each round, and
the whole of the recoil energy would have to be absorbed in that short
motion. Instead of this the gun alone is allowed to recoil several
feet and although the recoil energy is in this case greater than it
would be if gun and carriage recoiled together yet it is so gradually
communicated to the carriage that instead of a violent jerk we have a
steady, uniform pull, the only effect of which is to slightly compress
the earth behind the spade. In a well designed carriage the amount of
this pull is always less than that required to lift the wheels off the
ground by rotating the carriage about the spade.
The only motion of the carriage which takes place is that due to the
elastic bending and rebound of its parts under the cross strains set
up on discharge. These strains are inevitable since the direction of
recoil cannot be always exactly in the line of the resistance of the
earth behind the spade. This movement of the axis is known as jump
and must be determined by experiment for the individual piece in its
particular mounting.
The principal parts of the typical gun carriage are the cradle, a
device for mounting the cradle—called in the different models rocker,
pintle yoke, and top carriage, the trail, the wheels and axle. The gun
slides in recoil on the upper surface of the cradle and the cradle
contains the recoil controlling parts.
In the design of the carriage the constructional difficulty lies not
so much in preventing the carriage from recoiling but in preventing
the wheels from rising off the ground on the shock of discharge. The
force of the recoil of the gun, acting in the line of motion of the
center of gravity of the recoiling parts, tends to turn the carriage
over backwards about the point of the trail or center of the spade.
This force is resisted by the weight of the gun and carriage, which
tends to keep the wheels on the ground. The leverage with which the
overturning force acts is due to the distance of its line of action
above the center of the spade; the leverage with which the overturning
force acts is that due to the horizontal distance of the center of
gravity of the gun and carriage from the center of the spade.
It follows that the steadiness of the carriage for a given muzzle
energy may be promoted by four factors.
(a) Increasing the weight of the gun and recoiling parts. This reduces
the recoil energy.
(b) Increasing the length of recoil allowed. This reduces the
overturning pull.
(c) Keeping this gun as low as possible either by reducing the height
of the wheels, or by cranking the axle downwards. This reduces the
leverage of the overturning force.
(d) Increasing the length of the trail. This increases the leverage of
the steadying force.
The well designed gun carriage is one that combines these factors in
a practical way so as to give the greatest possible steadiness to the
carriage at the same time keeping within the limits of weight imposed
by the necessity of mobility.
Gun carriages are constructed so as to permit movement of the tube in
the vertical and generally in the horizontal plane. These two motions
may be made simultaneous if so desired by proper combination of the
two motions and the axis of a gun aligned in any desired direction
within the limits of motion of its mount. The two motions referred to
are designated as follows: (a) Rotation of the piece about a vertical
axis, its inclination with the horizontal remaining unchanged is
called “traverse.” (b) Movement of the piece in a vertical plane,
the direction of the horizontal projection of the axis of the bore
remaining unchanged is called “elevation.”
In order to permit of the two motions mentioned, gun carriages are
provided with mechanisms for giving the piece accurately controlled
motion in both elevation and traverse. The elevating gear of most
American guns is an application of the Chinese or Telescopic screw.
This gives a short assembled length with the necessary extended length
required for modern ranges. It also gives the necessary rapidity for
action. An entirely different mechanism is used in our howitzers and
American 75s. In this case motion is communicated to the rockers, which
carry the gun and cradle, through the engagement of worms with teeth
cut on the lower circular edge of the rockers, the latter being pivoted
on the cradle trunnions.
TRAVERSING MECHANISMS.
Movement of the gun in traverse is accomplished in two general ways.
(=a=) Pivot on the gun carriage axle.
1. The gun and cradle move around a vertical pintle or pivot placed
in a saddle or rocker which itself pivots on horizontal trunnions or
bearings usually attached to the axle. The rear end of the rocker forms
a seat or bearing on which the cradle and gun move in traverse. The
upper end of the elevating screw is also attached to the rear end of
the rocker. This allows the rocker and cradle to be moved together in
elevation and the cradle to move on the rocker in traverse.
2. The gun and cradle are mounted by means of horizontal cradle
trunnions on a top carriage. The top carriage moves in traverse around
a vertical pintle or bearing attached over the axle and between the
front end of the trail flasks. The rear end of the top carriage moves
on bearings attached to the top of each side of the trail flasks. This
allows the gun, cradle and rocker to move in elevation with respect to
the top carriage and the top carriage bearing the gun, cradle rocker
and elevating mechanism to move in traverse with respect to the bottom
carriage.
(=b=) The gun, cradle and trail move in traverse by sliding along the
axle of the carriage on bearings provided for that purpose. In this
case the trail spade is the point of pivot.
The first system is the one generally used on all American gun
carriages except the 155-mm Howitzer. It gives ease and speed in the
manipulation of the piece in traverse, but has the disadvantage of
rendering the carriage less stable in firing due to the fact that
the direction of recoil of the gun is in the direction of the line
joining the trail spade and the central pivot, _only_ when the gun is
absolutely in the center of its traverse. In all other positions, which
will be the usual case, there is a component at right angles to the
line of trail spade-central pivot which tends to throw the gun off from
its proper direction making it necessary to relay each time the gun is
fired. The second method is that in use on the French 75-mm gun and the
French and American 155 howitzer. While not so easy of manipulation and
giving less freedom of movement in traverse (about 106 mils) it still
has the advantage in that it gives greater stability in firing due to
the fact that the gun always recoils along the line through the trail
spade, perpendicular to the axle.
ELEVATING MECHANISMS.
There are two general classifications of gun carriages according to the
manner in which the laying for elevation is effected. The 3” gun is
an example of one type in which the total quadrant angle of departure
is laid off as one angle necessitating at each resetting of a range a
new resetting of the range bubble. It is evident that this militates
against both speed and accuracy in laying for elevation. The other type
is illustrated by the American 75 and by the French and British 75-mm
gun carriages which have what is called the “independent line of site.”
It differs from the other type in that there is placed between the top
carriage and the cradle an intermediate carriage or rocker arm pivoting
on the gun trunnions at one end, and carrying at the other the support
for the elevating device which gives to the cradle and the gun the
proper elevation for the range. This intermediate carriage or rocker
arm has a toothed edge geared with a pinion fastened to the trail
flask, by means of which, gun cradle and elevating device may be moved
in elevation without disturbing the relation between itself and the
gun, this being done by gears independent of the elevating mechanism.
An angle of site may be set off for the intermediate carriage and after
the bubble is centered this actuating pinion may be blocked and no
further concern be paid to the angle of site. A cannoneer therefore has
only to set his range accurately on the index to give the changes in
range ordered.
Elevation is accomplished by telescoping screws, by circular racks and
pinions, and by worms and arcs. Telescoping screws are good for from
15 to 20 degrees in elevation only, as for greater angles they become
rather too large and cumbersome. The elevating arc is attached on the
top, on the bottom or on the side of the cradle and, if possible, at
its center. When the latter is not possible, two arcs are generally
used to prevent torsional strains developing. Top racks are exposed to
fire, side circular racks or arcs interfere with the traverse, with the
placing of the sights, and with the general handling of the carriage.
The bottom of the cradle is perhaps the best location for the rack. As
in the traversing mechanism, a train of bevelled gears transmits the
power from the hand wheel to the elevating mechanism.
In both the traversing and elevating mechanism, strength, simplicity,
power, accessibility, non-interference and absence of lost motion are
the features sought. To secure these is one of the most difficult
things in gun-carriage design; for, since the traversing and elevating
are the last two elements considered, they must, therefore, be the
ground for modification and the means of adapting the great main
elements—namely, wheels, axle, trail, recoil mechanism and gun—into a
unit.
RECOIL MECHANISM.
The recoil system of the gun carriage consists of a recoil brake, a
counter recoil mechanism and a counter recoil brake. The function of
each part is apparent from its name. Various systems of checking recoil
on field guns have been used, among them may be mentioned; friction
devices including brakes on the wheel, inclined planes, pneumatic and
hydraulic brakes. All have either been superseded by the last named or
are used in connection with hydraulic brakes. The power of the brake
lies in the pressure produced in the cylinder through the resistance
to motion offered by the liquid flowing through apertures. By varying
the size of these apertures the braking effect may be controlled so
as to fulfill the conditions demanded by the service. In designing
the brake, the effect of the counter recoil system, angle of fire,
length of recoil, friction and pressure within the cylinder must all
be considered. In howitzers which are designed to be fired at high
angles of elevation, the recoil must be shortened to prevent the breech
striking the ground, a condition successfully met. Since the pressure
due to recoil is ultimately led into the ground through the trail and
spade, consideration must be given to the problem of the moment of
inertia about the trailspade. The tendency to turn over backward about
the spade as the center is offset by the amount of the weight of the
carriage with respect to the same point. This raises the question as to
how much pressure may be allowed to act to the rear; all of which must
be considered in designing the carriage. The counter recoil systems in
general use are two: spring or pneumatic. The former is illustrated in
our 3” and 75-mm field guns, the latter by the French 75-mm gun and
155-mm howitzer. Their purpose, of course, is to return the gun “into
battery” after it has recoiled on the carriage.
The recoil mechanism is a study in itself, of which there are two
schools—the advocates of the spring and of the air recuperators. Great
Britain, Germany and the United States have been the advocates of
spring recuperation and France of air recuperation. Great Britain and
the United States were of the spring school, undoubtedly, because of
the lack of a satisfactory air recuperating system; which is rather
strikingly proven by the fact that both countries have adopted air
recuperation since they have procured or developed satisfactory types.
Both schools have grounds for their position, however. The spring
school has in its favor simplicity of design and manufacture and ease
of replacement, which can be done in the field. On the other hand,
spring recoils have many breakages and greater weight combined with a
high replacement of weakened springs, the life varying from 3000 to
8000 rounds.
The air school has in its favor a high order of efficiency—smoother
action, general all around efficiency and less weight. But the air
recuperator is difficult to manufacture, costly, and when damaged must
go to the rear to be repaired; which, however, it seldom needs.
In mounting the spring recuperator, the most recent practice has
separated the recoil mechanism from the springs in order to distribute
the piston rod pull, thus preventing whip and allowing easy access to
the various parts for replacement, refilling and repairs. In order to
lower the center of gravity, the gun is slung under the recoil cylinder
with the two spring recuperators below and on either side.
Air recuperators are invariably located below the gun for protection
and because of their large size and shape, which adapt them for
attaching the elevating mechanism.
[Illustration: BATTERY OR FIRING POSITION
IN-BATTERY OR FIRING POSITION
HYDRO-SPRING RECOIL SYSTEM
HYDRO-PNEUMATIC RECOIL SYSTEM WITH FLUID IN DIRECT CONTACT WITH THE
AIR]
In either system, the recoil is taken up by means of oil or glycerine
and water passing through an orifice created by a slotted piston
passing over ribs of varying height, or through a valve on the pressure
side of the piston, or by a solid or perforated piston passing through
a perforated intermediate cylinder.
The latter type is particularly adapted to variable recoil guns as the
intermediate cylinder can be rotated, thus throwing varying orifices
into position for the flow of oil.
Counter recoil is accomplished by the springs or by the air pressure in
the hydro-pneumatic system, in which the air pressure is sufficient to
hold the gun in battery at all elevations and is built up at recoil.
In all counter recoil systems, it is necessary to insert a buffer to
take up the remaining energy of the springs or air pressure so as to
bring the gun into battery without appreciable jar. Numerous types have
been developed and perfected.
The counter recoil brake or buffer in our 3”-gun is a slightly tapered
bronze rod, tightly fitting in the cylindrical bore of the piston rod.
The retardation caused by forcing the oil in the piston rod out through
the small clearance between the buffer and bore of the piston rod eases
the return to battery without jar to the gun, which has been forced
back by the counter recoil springs.
The physical law that action and reaction are equal has a peculiar
emphasis when applied to the firing of a piece of high powered
artillery. The force exerted to throw a heavy projectile 7 miles or
more from the muzzle of the gun is toward the breech of the weapon in
its recoil. How some of these forces are handled safely and easily by
mechanical means are almost beyond the mind’s grasp. Not long ago a
touring car, weighing two tons, traveled at the rate of 210 miles an
hour along a Florida beach. Conceive of such a car going 337 miles an
hour—which is much faster than any man ever traveled; then conceive of
a mechanism which would stop this car, going nearly six miles a minute,
stop it in 45 inches of space and one-half a second of time without the
slightest damage to the car. This is precisely the equivalent of the
feat performed by the recuperator of a heavy howitzer after a shot.
SHIELDS.
Although cover for the cannoneers had been used off and on since the
invention of guns, it had fallen into disuse until it was firmly
established as an essential feature by the French on their 75-mm
in 1897. All modern field guns have such protection both for the
cannoneers and for the delicate parts of the material which would
be damaged by shrapnel balls or shell fragments. The shield is made
of hardened steel capable of withstanding the impact of a bullet
of a service rifle at a 100 yds. range at a standard velocity. For
convenience the shield is divided into three parts known as the top
shield, main shield and apron, with suitable ports equipped with
shutters for the line of vision from sights. The main is fastened to
the axle and is rigid. The apron is hinged to the main shield or the
axle-swinging forward for the traveling position. The top shield is
fastened to the main shield by hinges and swings forward and downward
for traveling position.
SIGHTS.
The sights serve three important functions. They improve the vision of
the gunner and lay the gun in elevation and direction. The simplest
sight is the one over the line of metal which lays for direction only,
the second is the tangent sight mounted on a range arc centered on the
axis of rotation in elevation usually having a deflection scale to
correct for drift and to lead the moving target. This unit lays for
range and direction. The last sight is the telescopic or panoramic
sight which is mounted on a range arc and lays for direction only,
it is the unit for indirect fire, laying for direction, and markedly
improving the vision of the gunner. The latest model of the American
panoramic sight is superior to any in existence.
For precision in indirect fire, practically all carriages are equipped
with some form of range quadrant, containing a means of setting off
the angle of site and the range angle. All instruments are equipped
to compensate for difference in wheel level. The British carriage
automatically corrects for deviation, simplifying the firing date by
that element.
WHEELS.
The height of the wheels affects the draft, weight, clearance, and
stability of the carriage both as to road stability—i. e., low center
of gravity; and firing stability—i. e., the overturning movement
about the end of the trail. It is now believed that entirely too much
stress has been laid on road clearance. Reducing the height of wheel
reduces the weight and road clearance, lowers the center of gravity and
increases the firing stability; but it also reduces the angle of gun
elevation unless the trail is shortened. Firing stability in general is
increased by adding to the weight of the gun, lengthening the recoil,
slinging the gun as low as possible and lengthening the trail. The less
the height of the wheel exceeds four feet the better, despite the poor
draft feature, which is compensated for to some extent by a reduction
in weight and turning radius.
AXLES.
Axles are straight or of the offset type. The straight axle is stronger
for its weight. The drop axle allows the center of gravity to be
lowered.
TRAILS.
Most modern trails are of the sectional built-up type. Some, however,
are of tubular and telescopic. The most variable portion of the trail
is the spade. It consists of two parts, the spade proper and the float.
The former prevents recoil, the latter the burying of the trail. The
spades proper are of three types: the fixed, as in the French 75-mm;
semi-fixed, as in the 155-mm howitzer; and driven, as in the Deport and
American 1916 75-mm. Each has advantages and disadvantages. The driven
spade is considered essential for the split trail carriage, as the
latter has no means of seating itself; and should one spade take, and
not the other the carriage might be damaged when the gun is fired at an
extreme traverse.
Split trails introduced a novelty in field gun carriages, in that a
compensating device became necessary to adjust for the difference in
ground level of the two spades.
CONCLUSION.
Guns are designed to function in a certain way. They are not
temperamental. They follow absolutely and certainly fixed mechanical
laws. If they fail there is a reason and it can be remedied. Certain
parts are given certain shapes and forms, are machined to nice
adjustments, and in taking down and assembling them, brains and
dexterity are the tools to use rather than force and sledge-hammers.
Learn from your text when and how to apply force and above all when not
to use it. Treat these guns as you would a friend on whom you know you
can depend. They will not fail you.