HMS Colossus was the first ship to carry the new generation of breech loading main armament - four 12 inch guns Mk II made by the Woolwich Arsenal. This design, which used a mixture of steel and wrought iron was not a success. Although Colossus was launched in 1882, it was not until 1886 that she received her full complement of 12 inch guns. In 1887, her sister ship, "Edinburgh" suffered a burst gun, and Colossus was ordered not to fire her guns until a replacement was constructed. In this section we look at how the replacement (Mk IV) 'built-up' gun was constructed.

As far back as the 1850's Sir William Armstrong had designed guns that were build up from concentric hoops or tubes each shrunk onto inner tubes to put these tubes into a state of compression. Therefore when a propellent charge was ignited, the pressure had first to overcome the compression imposed by the outer tube before stressing the inner tube. This approach of shrinking outer tubes onto an inner resulted in much lighter gun construction, since less metal was needed.

12 in Breech Loading Gun of 45 Tons Mark IV

The 12 inch, 45 ton Mark IV was designed by Armstrong's Elswick works. The gun consists of:-

- an inner or A Tube

- a first layer, the breech-piece or B tube, and C, D and E hoops

- a second layer, the trunnion ring (H hoop), F and G hoops, the I hoop extending the whole length of the powder chamber, and the J hoop.

- a third layer consisting of the K hoop, L hoop, and M hoop.

All parts are of steel - cast, forged, hardened in oil and 'boiled'.

Manufacture

The inner barrel is cast solid, and then drawn out under the hammer to the required length. It is then rough turned and the properties of the steel are tested. After passing the tests, it is rough bored, hardened by heating and cooling in oil, and then 'boiled' in oil and tallow to reduce any abnormal strains (tempering).

The next stage is turning and second boring after which it is ready for building up.

The A tube measures 303 inches by 18 inches diameter (7.69 m x 45.7 cm), and must be machined to within a few thousandths of an inch to ensure the outer layers can apply the correct amount of compression when shrunk on.

The breech-piece and long hoops are cast and forged solid, (except in certain cases when they are forged hollow on a mandrel), rough turned and tested. After testing the centre is trepanned out and they are then hardened and boiled, second bored and turned. The inner diameter of the hoop will be machined to a few thousandths of an inch less that the outer diameter of the tube that they will be shrunk onto, in order to put that tube under compression.

Short hoops are cast solid and a hole punched through them; they are then forged on a mandril, round bored, tested, hardened, boiled, and then second turned and bored.

The trunnion ring (H) is cast solid and drawn under the hammer into the proper shape.

Construction

1.

The steel barrel and breech-piece, having been prepared for each other by turning and boring to corresponding dimensions, are shrunk together as follows:-

The breech piece is heated until it is sufficiently expanded to fit easily over the breech end of the steel barrel, which is placed upright in a pit ready to receive it. The breech-piece is then raised and dropped over the steel barrel, up which a stream of water is forced to keep it as cool as possible during the process of shrinking.

2.

The gun is repositioned in the pit with muzzle uppermost, and the C, D and E hoops are shrunk on in a similar manner, after which the gun is returned to the lathe and fine turned to receive the trunnion ring and hoops.

3.

The F hoop is next to go on to lock the breech-piece to the C hoop by means of a bayonet joint.

After wedges have been driven in, the G hoop is shrunk on to keep them in place.

4.

The gun is repositioned in the pit with breech end uppermost. The Trunnion ring, I and J hoops are next shrunk on in succession.

The I hoop specifically re-inforces the breech along the length of the powder chamber.

The gun is returned to the lathe and fine turned to receive the next layer of hoops.

5.

The K hoop locks the trunnion ring and the I hoop.

6.

The M hoop locks the I hoop and the J hoop, and holds the L hoop in position.

Summary.

It will thus be seen that the longitudinal strength is provided by the breech-piece or B tube, which carries the breech screw at one end and is locked to the trunnion ring at the other.

The second layer of hoops also contribute to the longitudinal strength, since the I and J hoops are locked to the trunnion ring and to one another by the hoops K and M, the J hoop being hooked on to the rear of the breech-piece.

The A tube is prevented from moving forward by a shoulder butting against the C hoop, which is also locked to the trunnion ring by the hoop F.