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There are three designs of 4-inch BL guns.
The Mark VII is a high velocity gun, having a M.V. of 2,852 f.s. It is mounted as an anti-torpedo craft gun in large ships, and in the main armament of smaller ships. The Mark VIII and VIII* are medium velocity guns, having a MV of 2,287 f.s. They are mounted in light craft where heavy deck strains cannot be allowed.
The following are the chief particulars relating to the guns:-
(Nos 501-506 only)
|Material||steel wire construction||steel wire construction||steel wire construction|
|Weight||42 cwt||26 cwt||26 cwt|
|Capacity (cubic inches)||600||298||298|
|Twist||Uniform 1 in 30||Uniform 1 in 30||Uniform 1 in 30|
The charge for the Mark VII gun consists of Cordite MD size 16, 9 lbs. 5 15/16 ozs., and for the Mark VIII and VIII* guns the charge is 5 lbs 4 ozs. of the same size cordite. The following projectiles (3 calibre head), weighing 31 lbs, are supplied for use with these guns:-
The Mark VII gun is built up of steel tubes and wire, and consists of an inner A, A and B tubes, a series of layers of steel wire, jacket, breech bush and breech ring. The inner A tube is driven into the A tube and secured longitudinally by means of corresponding shoulders, and a steel breech bush, which is screwed into the A tube at the rear, and is also threaded internally to receive the breech screw.
With a view to diminishing choking of the bore cannelured rings are placed between the shoulders of the A and inner A tubes at the points in the plate marked A, B &c.
Successive layers of steel wire are wound round a portion of the A tube, the ends being secured to steel rings provided for the purpose.
The B tube is shrunk round the A tube immediately in front of the wire and extends to the muzzle.
The jacket fits over the wire and part of the B tube, and is secured longitudinally by corresponding shoulders on the B tube, and by the breech ring, which is screwed on to its rear end and also fits over the A tube.
Projections are formed on the breech ring for the attachment of the piston rod, running-out rod and carrier arm. When worn the gun is repaired by the insertion of a new inner A tube and breech bush.
The Mark VIII gun differs in construction from the Mark VII gun in that no inner A tube is fitted, and a breech piece is shrunk over the rear end of the A tube, and into this the breech bush is screwed.
The jacket is furnished with a longitudinal projection on either side, which forms guides for the gun in the cradle, and also bear the weight of the gun. Cannelured rings are not used.
When worn the gun is repaired by boring out a part of the A tube and inserting a tapered liner.
The breech is closed by a breech screw on the Welin System, the mechanism being of the single motion type.
The flange at the rear of the breech screw has a long arm projecting from it, in which a cam groove is cut, for the purpose of giving the necessary rotation for locking and unlocking. When in the unlocked position and swung away from the breech face, the breech screw is retained against rotation by a spring plunger catch.
The breech screw is bored out in the centre to receive the axial vent and carrier stem, the rear portion being threaded with an interrupted thread.
The carrier for supporting the breech screw is pivoted by the hinge bolt on the right side of the gun. The stem of the carrier on which the breech screw is mounted is provided with interrupted threads to correspond with those in the breech screw, and is bored to receive the axial vent.
The crank pinion is pivoted on a horizontal axis on the carrier, and has bevel teeth which engage in bevel teeth on the hand lever pinion. A roller pion projects forward from the outer end of the crank and engages with the groove in the long arm of the breech screw. The form of the groove and its position relative to the crank pinion is such that the maximum power is exerted when setting the obturator and a locking point is obtained when the breech is closed.
The hand lever is mounted on a pivot on the carrier so as to swing in a horizontal plane and to lie close up to the breech when the mechanism is in the locked position. In this position the lever is retained by a catch.
The hand lever pinion which actuates the crank pinion is secured by feathers on the hand lever axis.
Obturation is effected by means of a plastic canvas-covered pad (the latest pattern is reinforced by woven brass wire) with a copper front protecting disc and steel front and rear rings; this obturator is held at the front of the breech screw by the mushroom head of the axial vent.
The stalk of the axial vent passes through the breech screw and carrier, being secured to the former by a sleeve bearing against the inner face of the breech screw. This sleeve is keyed longitudinally to the interior of the carrier stem and to the axial vent. In rear of th sleeve is a spiral spring, a rear washer keyed to the carrier and a retaining nut screwed on the stalk of the axial vent, the latter securing the whole system.
At the rear end of the axial vent interrupted collars are provided for holding the box slide.
The tube chamber is made to take the large V.S. Tubes. To limit the longitudinal movement of the axial vent the total movement of the spring is restricted to .05 inch, this being the maximum possible clearance in any position of the breech mechanism.
Provision is also made for preventing the slacking back of the axial vent nut. Two projections are formed on the rear face of the nut which engage with similar projections on the box slide, thus preventing rotary movement of the nut whilst the box slide is in position. The box slide is provided with one large collar, which makes it impossible to turn it if it is not in its proper position on the vent axial. If the retaining nut is more than one quarter of a turn from its final position the box slide cannot be shipped owing to this provision. If the nut is less than one quarter of a turn from its final position, the projections on the box slide will engage with those on the nut and screw it home.
The firing gear comprises the box slide "F", with separate percussion and electric locks "Y" and "W" respectively, with are interchangeable on the box slide.
The gun is mounted in a special cradle similar to that of the PII* mounting, but provision is only made for one sight on the left side of the cradle.
The elevating arc is also longer than that of the PII* mounting in order to obtain the 20º elevation required in the field mounting.
The elevating gear, which is single handed, is similar to that of the PII mounting. It is bolted to the left side of the trail.
The trail is built up of steel plates and consists of two side brackets, top and bottom plates and transoms, the side brackets being fitted with trunnion bearings and cap squares.
The axle tree is a hollow steel forging, tapered at either end to receive the wheels.
These are of 56" diameter, fitted with 5" steel tyres, and are secured to the axle tree by a collar and cotter pin.
The collar has a number of notches of varying depth on its outer edge, and as the hub of the wheels wears away the ensuing play can be taken up by shifting the pin to pass through a shallower notch.
The end of the axle tree is covered by a dome-shaped cover which screws onto the wheel, and by means of a hole, fitted with a screw plug on the inside of the nave, this space around the end of the axle tree can be filled with oil.
A leather "L" washer is fitted between the wheel and axle to prevent the escape of oil or ingress of dust and grit. The track of the wheels is 5 feet 10.5 inches.
The trail can be moved laterally on the axle tree to the extent of 2 ½º each way, the sleeve in the trail, which works over the axle tree, being elongated to allow of this angular movement.
The movement is effected by means of a traversing screw actuated by a hand wheel on the right of the trail, and a traverse nut carried on the axle tree on a link, which allows it the necessary angular movement as the trail pivots about its rear end.
The traversing screw, supported by the side brackets, is fitted with ball bearing rings and with adjusting and check nuts on the left, the latter being covered by a removable metal cap.
A travelling link is fitted on the left, by means of which the trail, when in the central position, can be secured to the axle tree, so taking the strain off the traversing screw and nut when on the march.
With the sights at zero they will butt against the trail at an elevation of the gun of 14º, and to prevent their being damaged in this manner a spring top is fitted, which, as the gun approaches this elevation, is forced into a hole in the elevating arc, so locking the gun until the stop is agin removed by hand. A brass lengthening piece is fitted at the bottom of the arc to prevent the stop getting underneath it when the gun is given its maximum depression.
The bottom of the rear end of the trail is shaped so as to act as a spade, and, when firmly embedded in the ground, combines with the length of the trail to give a steady carriage without jump. It is also fitted with trail handles at either side, and a roller of 18 inches diameter and 9 inches tread, upon which the carriage travels. This roller can be turned up out of the way for firing, and, if found convenient, can be removed entirely and a trail eye fitted.
The roller is fitted with an iron handle with a wooden cross-piece, by means of which the carriage is guided.
A brake is fitted to act simultaneously on both gun wheels. It is worked by a system of rods on the under side of the trail actuated by a handle at the front of the carriage or by another in rear of the right gun wheel, five pairs of Belleville washers forming a spring to take the brake off when these handles are eased back.
The brake is set hard up before firing and takes the place of drag shoes.
A tank is fitted in the rear end of the trail and is used for stowing spare gear.
Sighting telescopes in steel boxes and a clinometer in a leather cas are secured in suitable positions on the trail.
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