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The Armstrong Gun

Part 2: The RBL Armstrong 12-pr Field Gun

The 12-pr has been chosen for study in detail because it is the most interesting, It probably saw more action in New Zealand than any of the others, and because it is the only piece of which a near-complete example survives in this country; it may be seen at Te Awamutu Museum (1).

The description of the piece (with the exception of the "grip" which is peculiar to the 12-pr), is typical of all the Armstrongs. Except for size other differences in detail are not significant.

Field carriages for all equipments except 20 and 110-prs (which were restricted to coast and naval roles), were copies of their smooth-bore counterparts.

The gun was of 3-in (76.2mm) calibre firing a projectile weighing approximately 12 lbs (5.44 kg) - shot and shell differed somewhat in weight - with a muzzle velocity of 1239 fs (372 ms). It was mounted on a carriage very similar to that of the smooth-bore muzzle-loading 9-pr field gun it superseded, but the gun itself differed radically from any other piece of ordnance then in existence.

Armstrong had long realised that the cast iron which sufficed for SBML guns would not stand the additional stresses produced by rifling. Steel he also discarded as forgings large enough for his purpose often contained flaws in those days. He therefore selected wrought iron, employing the "built-up" method of construction.

In this, a tube forming the bore is the foundation of the piece. A second tube of internal diameter too small to fit over the first is heated until it expands to the stage where it will fit. On cooling the outer tube shrinks, gripping the inner very tightly indeed. The process is repeated with additional tubes of varying length until the piece is built up to the required size and shape.

The assembly is said to be "prestressed", for the outer layers of metal remain in a state of tension, while the inner remain in a state of compression. This means pressure from the propellant gases must first overcome the tension/compression effect before actually stretching the metal itself. Thus the piece can be made smaller and lighter than if it were machined from the solid.

Rifling was of the "polygroove" (2) type which in the 12-pr consisted of 38 shallow grooves with a uniform twist of one turn in 38 calibres i.e. 114 inches (2896 mm). Such a system was and still is of sound design for it enables a distribution of stresses and wear most favourable to the life of the bore. Indeed it is a tribute to Armstrong's engineering skill that his rifling closely resembles that of modern times, both in form and principle.

The 12-pr bore possessed a unique feature not found in any other of his guns. For a short distance at the muzzle the bore was slightly reduced in diameter to form what Armstrong called a "grip", intended to ensure precise centring of the projectile as it left the gun. It also imparted a better ballistic shape by partially swaging down or smoothing the lead coating, thus reducing air resistance.

In other words in 1859 there existed a practical application of the "squeeze-bore" principle thought by many to have been a World War II invention. "Truly", saith the sage, "There is nothing new under the sun!"

The breech mechanism comprised two main parts, a breech block and a breech screw operated by a weighted crank handle on its rear end. The block moved vertically in a slot cut in the breech end of the piece; it was lifted bodily out for loading and replaced for firing. Called the "vent piece" it incorporated the vent into which was inserted the 'T' friction tube, the flash from which ignited the propellant charge of gunpowder in the chamber.

Obturation (3) was effected by a conical copper-ringed plug formed on the front face of the vent piece fitting into a copper-bushed breech opening shaped to receive it; a tin cup (4) inserted in the chamber behind the cartridge assisted. The hollow breech screw (through which the gun was loaded), fitted over a projection on the rear of the vent piece. Thus when tightened the screw ensured the plug on the front of the vent piece made a tight fit in the chamber, and at the same time prevented the vent piece from blowing out.

The gun was fitted with a tangent sight (5). The basic structure consisted of a hexagonal bar which could be fitted into a socket on either side of a carrier attached to the breech. It was graduated in yards up to 3400 (3100 m). Surmounting the top was a cross-head with a notch (the backsight), and a scale upon which deflections for line up to 30' right or left could be set. The sockets for the bar were inclined at an angle of 216' to the left, so that as the bar was raised a mean correction for drift (6) was automatically applied.

Elevation was applied to the gun by means of a screw, one end of which was attached to the bottom of the breech piece. The other end passed throogh a ball threaded to receive it working in a socket fixed to the trail. Four handles were fitted to the ball, operation of which elevated or depressed the gun.

Although Armstrong made use of existing carriages upon which to mount his guns he introduced one very important innovation, probably a world first. His carriage was fitted with a traversing arrangement consisting of a metal saddle incorporating trunnion bearings in which the gun was secured by capsquares (7). This saddle slid in dovetailed slots in trunnion plates on the carriage, and was traversed by means of a lever pivoted on the trail. The lever in turn was worked by a traversing screw running horizontally across the carriage, and fitted with a handwheel. Total traverse, known as "top traverse" to gunners, was 1½ right and left - not very much by today's standards, but of inestimable value to the old-time layer (8) .

A device to control recoil was fitted to prototype field carriages but not to production models. Why this was so was not recorded; probably the additional weight involved was not acceptable. Some control was exercised by placing a skid pan under one of the wheels and hooking on to it a chain, the other end of which was fixed to the trail. In any case the recoil was easily manageable, the "run-up" (9) after firing being less than a metre.

Ammunition was as revolutionary in character as the gun. Shot and shell were of iron cast in moulds, machined to size in the lathe, and coated from base to shoulders with lead. When the gun fired the projectile was forced through the bore, the lead filling up the grooves of the rifling to make a gas-tight fit. The rifling at the same time imparted the spin necessary to stabilise it in flight. Thus the Armstrong gunner was not beset by the evils of windage (10) .

Armstrong made shot as well as common (11) and shrapnel (12) shell, types with which the authorities were familiar, but his speciality was "segment". This consisted of a body fitted with segment-shaped pieces of cast iron built up in layers around a cylindrical cavity in the centre. In the cavity was placed a small bursting charge of powder, merely enough to break up the assembly; the fragments depended for killing power upon the remaining velocity (13) of the shell which broke into about 200 pieces.

A metal "time and concussion" fuze, the first of a long line of similar types, screwed into the nose of the shell. It could be set to burst the shell on the trajectory as with shrapnel, or set to zero to burst it at the muzzle, thus producing the same effect as case or cannister. Alternatively the timing mechanism could be by-passed to allow the shell to burst on impact.

Such a weapon cannot be compared to a modern high explosive shell, but in the hands of a skilful gunner could cause many casualties. Its effect upon troops standing shoulder to shoulder in time-honoured 19th century style can well be imagined.

Note well the use of the term "weapon", for the shell is the weapon of the artillery, the gun merely the means of putting it on the target. In recognising this principle Armstrong devoted as much care to the design of his projectiles as he did to his gun.

Cartridges were still serge bags filled with gunpowder, destined to remain the most common propellant for the next 30 years. The most significant change was in weight - 1lb 8oz (63Ogm), against 3lbs (1.36kg) for the old SBML 9-pr field gun which dissipated much of its power overcoming windage. See Table 1.

A special slow-burning powder of greater density and larger grain than the service LG was developed for the Armstrongs because the "shot start pressure" (14) was obviously much higher than in the smooth-bores with their loose-fitting shot and excessive windage. Use of the faster-burning service powder would have generated dangerous pressures.

Ignition of the charge was by copper 'T' friction tube designed by Col Boxer RA, and adopted in 1853, so-called because of its shape. Filled with mealed powder it was inserted into the vent after the breech was closed. The top of the 'T' held a copper friction bar roughened on both sides and smeared with a flash-producing composition containing potassium chlorate, sulphur, and antimony sulphide. The protruding end of the friction bar was formed into an eye or ring to which the hook of the firing lanyard was attached. Pulling the lanyard initiated the composition, the flash from which progressed through the mealed powder and on to the charge, thus firing the gun.

Some idea of the performance of the RBL 12-pr may be obtained from Table 1 below in which it is compared with the SBML 9-pr it superseded.

TABLE 1 (For 28 Calibre Gun)
Weights Mean Deviation
9-pr SBML133 lbs190016009.1147
12-pr RBL
28 cal
81 lb 8 oz250012390.823

Weights of carriages have not been given as they were virtually identical.

No data is available for the longer ranges, but the Secretary of State for War reporting to Parliament in 1859 stated the 12-pr was "seven times more accurate at 3000 yards than the 9-pr was at 1000".

Although capable of longer ranges the 12-pr was only sighted to 3400 (3100 m). Even this was more than necessary for 19th century tactical thinking failed to keep pace with improvements in armament. In set-piece battles the short range slaughtering match continued to be the most-favoured method of deciding the issue.

The first 12-pounders made were of 28 calibres, i.e. the piece was 84 inches (213.36 cm) in overall length, not including the breech screw. Then at the request of the Navy a 24 calibre gun was introduced, eventually becoming general issue although it was not as efficient as the longer piece. The two guns were almost the same weight and looked very much alike, except the 28 calibre was a foot (30.5 cm) longer in the chase (15). Later still the "grip" at the muzzle was dispensed with, probably to make manufacture easier, and caused a further drop in performance. However, neither modification aroused much comment; tactical mentality was still of the smoothbore variety.

In Part III we shall see what happened in New Zealand with 12-prs in the hands of the Royal Artillery.

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1. This gun, a 12-pr 28 calibre. was on loan to the Queen Elizabeth II Army Memorial Museum, Waiouru, for six months following the opening of the Museum on 15 October 1978. Return

2. "Polygroove" simply means "of many grooves". Return

3. See Note 8 Part 1 for definition. Return

4. The tin cup behaved in much the same way as a modern cartridge case. Pressure from the propellant gases expanded it against the walls of the chamber. Return

5. Basically a tangent sight consisted of a foresight F attached to the gun and a hindsight H on a bar movable in a socket G fixed to the gun so that HG was always perpendicular to the axis of the piece. On the elevation being ordered, the hind sight was pulled out the appropriate distance in its socket so that the angle HFG formed was that required. The gun was then laid over the sights H and F so that the line of sight HF was on the target. The gun was thus given the required amount of elevation above the line of sight. From the trigonometry of the triangle HGF, since the base GF is fixed, HG must be graduated according to the tangent of the elevation HFG. Hence this angle was called "tangent elevation".


6. See Note 6 of Part 1. Return

7. Holding-down devices. Return

8. The layer "lays" the gun, i.e. points it in the direction of the target, and elevates it to achieve the necessary range. Return

9. Also known a "run-out". When the gun fired the carriage moved back on its wheels. The detachment was then ordered to "run-up", i.e. manhandle the gun and carriage back to the firing position. In a modern equipment the carriage remains stationary, recoil and run-out being controlled by buffer and recuperator respectively, which form an "elastic link" between gun and carriage. Return

10. See Note 1 of Part 1. Return

11. "Common shell" was the term given to shell filled with gunpowder. Return

12. Named after Lieutenant (later Major General) Henry Shrapnel RA who invented it in 1784. It underwent trials in 1792, was adopted in 1804, and officially named after its inventor in 1852! Originally called "spherical case" it started as a spherical shell filled with musket balls and a small bursting charge, just enough to split open the assembly and release the contents. Shrapnel depended for its effect upon the remaining velociy (Note 13) of the shell, being timed to burst at a height above troops calculated to give the balls maximum spread and killing power. It survived until the outbreak of the 1939-45 War, during which the term shrapnel was often incorrectly used to describe splinters from high explosive shell. Return

13. The velocity of the projectile at any point on the trajectory. Return

14. The pressure of propellant gas necessary to start the projectile on its travel up the bore. Return

15. In the Armstrong and later equipments that part of the gun between the trunnion ring and the muzzle. Return

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