EOC 12-inch L/27.5 43-ton gun

Elswick Ordnance Company

Industry	Naval artillery manufacturing
Founded	19th century
Founder	William Armstrong, 1st Baron Armstrong
Headquarters	United Kingdom
Products	Naval guns, including the EOC 12-inch L/27.5 43-ton gun
Owner	William Armstrong, 1st Baron Armstrong (predominantly)

The EOC 12-inch L/27.5 43-ton gun represents a rifled breech-loading naval artillery piece developed in the early 1880s by British and Spanish forces. Its inception likely stems from the challenges encountered by the Royal Arsenal at Woolwich during attempts to produce the heavy BL 12-inch Mk I – II naval gun. Dimensionally, the EOC 12-inch L/27.5 closely matched the 12-inch Mk I – II but initially seemed to offer less power than its Woolwich counterpart. Ultimately, however, the designers at the Elswick Ordnance Company (EOC) demonstrated superior engineering in their design. At least one unit was exported to Spain.

Context

Main article: BL 12-inch Mk I – II naval gun

In the mid-1860s, specifically 1864–65, Britain reverted to muzzle-loading artillery, which appeared sufficient at the time but by 1878 was recognized as an obsolete approach. The Elswick Ordnance Company (EOC), predominantly owned by William Armstrong, 1st Baron Armstrong, had seemingly arrived at this realization earlier. In February 1877, EOC conducted trials with its breech-loading EOC 12-inch L/23.5 model. Later, in September 1878, it tested a chambered BL 6-inch 80-pounder gun, a medium-caliber weapon that integrated breechloading with chambering techniques and yielded groundbreaking performance.

During this period, 'chambering' referred to increasing the powder chamber's diameter relative to the barrel's bore. This design permitted larger propellant charges while maintaining reduced peak pressures within the barrel. For example, in October 1876, the Royal Arsenal at Woolwich trialed the chambered RML 12.5-inch 38-ton gun. Similarly, Krupp’s prototype 24 cm MRK L/25.5, first assessed in December 1878, featured chambering and delivered power comparable to the RML 12.5-inch but weighed only 18 tons.

Tests conducted by Krupp in Meppen in August 1879 showcased their 24 cm MRK L/25.5 prototype alongside various other artillery pieces. These demonstrations made it evident to the British public that Woolwich Arsenal’s technology lagged significantly behind that of Krupp and Elswick’s offerings. The arsenal itself had already reached this conclusion prior to public awareness. Achieving equivalent firepower necessitated extending barrel length and adopting rifled breech-loading mechanisms simultaneously—a transition that led to significant complications during development of the BL 12-inch Mk I – II naval gun. Work on this particular 12-inch, 43-ton weapon began around 1879 or possibly as early as 1878.

Development

EOC 43-ton gun, 1881

Following the March–April 1880 United Kingdom general election, the Liberal Party secured victory, resulting in Gladstone assuming the role of prime minister. Approximately a year later, George Trevelyan, serving as Parliamentary and Financial Secretary to the Admiralty, recounted that the incoming administration’s initial measure was to devise an alternative to the Woolwich gun.

In fact, on 24 May 1880, the government formally requested Sir William Armstrong of the Elswick Ordnance Company (EOC) to deliver a 43-ton breech-loading gun to Woolwich for testing purposes. Armstrong agreed but required a ten-month period for completion. Production spanned from June 1880 until August 1881.

This particular 12-inch artillery piece was not unique among those produced by EOC. In October 1881, American Colonel S. Crispin visited the company and observed two additional 43-ton breech-loading guns of the standard Armstrong design—featuring a steel tube reinforced by coiled jackets—under construction.

When HMS Conqueror was commissioned in September 1881, Trevelyan’s account regarding the procurement of the EOC gun was reiterated. It was also reported that two such guns would be installed aboard Conqueror. However, it remains uncertain whether these guns were actually manufactured by EOC.

An EOC 43-ton gun commissioned by Spain

The Cadiz gun in 1882/3

In June 1880, a contract was finalized between EOC and the Spanish government for one 10-inch and one 12-inch gun, both slated for delivery within seven months. The combined cost amounted to 418,000 pesetas or equivalently 175,600 Austrian florins. The 25.4 cm gun arrived in October 1881. However, by June 1882, delivery of the 12-inch gun had yet to occur.

During testing conducted in Cadiz in December 1884, it was mentioned that production of this gun commenced in 1882. This suggests that the Spanish gun was manufactured subsequent to the two guns under construction in 1881.

Technical specifications

EOC 12-inch L/27.5 43-ton gun in Cadiz, 1883

The early description of the 12-inch 43-ton gun indicated an overall length of 331 inches (8.407 m) (L/27.5), with a bore length measuring 312.2 inches (7.930 m) (L/26). Its caliber measured 12 inches (0.3048 m), and it featured rifling composed of fifty grooves. The Spanish variant exhibited a progressive twist rate corresponding to L/45.

It remains unclear if the version utilized by Spain differed substantially from the two guns produced in 1881. Shortly before testing in January 1883, specifications were reported as follows: total length of 8.25 m; bore length of 7.92 m; mass of approximately 44,000 kg; projectile weight at 317.5 kg; maximum powder charge of 158.55 kg; service charge at 149.5 kg; and muzzle velocity with maximum charge reaching 595 m/s.

The differing weight attributed to the Spanish model appears to result from rounding conventions: while a nominal weight of 43 long tons converts to approximately 43.7 metric tonnes, this figure is often rounded up to about 44,000 kg. Likewise, the total length measurement of 8.25 m likely stems from similar approximations. As with other nations’ practices, Spain tended to round dimensions to whole centimeters—thus transforming a precise caliber of 30.48 cm into a nominal designation of 30 cm. Alternative sources cite measurements including an overall length of 8,382 mm, bore length of 7,925 mm, and weight near 43,688 kg. Upon final delivery to Cadiz in 1883, the gun’s weight was documented at approximately 44,350 kg.

The structure of the Spanish gun consisted of a forged cast steel inner tube encased by two layers of coils: the first comprised six segments extending up to the muzzle; the second included four coils, one featuring specialized trunnions.

The initial version’s powder chamber measured 86.25 inches (2.191 m) in length with a diameter of 14.3 inches (0.3632 m), yielding a volume capacity of approximately 13,178 cubic inches—dimensions notably longer and narrower but also more capacious than those found in competing Woolwich guns.

Performance

The initial model of the gun utilized a service charge of 330 pounds (149.7 kg) to launch a projectile weighing 700 pounds (317.5 kg) at a speed of 1,835 feet per second (559 m/s). The internal pressure recorded in the gun reached 17.1 tons per square inch (2,606 atm). In comparison, the Woolwich gun achieved a similar outcome while employing a reduced service charge of just 286 pounds (129.7 kg). This charge propelled a projectile of 720 pounds (326.6 kg) to a velocity of 1,855 feet per second (565 m/s), with pressure estimated to be equivalent.

Based on this data, American Colonel Crispin deduced that the Elswick gun was less efficient than its Woolwich counterpart. However, another expert observed that Armstrong’s design would endure less longitudinal stress under identical pressure conditions. Illustrations reveal that the EOC gun’s barrel was spirally reinforced up to the muzzle.

The contract for the Spanish gun required initial testing in England. The specifications demanded firing three 317.5 kg projectiles using a charge of 320 pounds (145.1 kg), achieving an average velocity of 1,950 feet per second (594 m/s). Additionally, peak pressures were not to exceed 20 tons per square inch (2,722 atm). During testing, the gun only reached a velocity of 1,889 feet per second (576 m/s), despite increasing the charge to 350 pounds (158.8 kg). Comparable results were obtained during a Woolwich test involving another 43-ton gun built for the Spanish government.

In late January 1883, a second evaluation of the Spanish 43-ton gun occurred in Cadiz, attended by a special commission and Major Jones representing EOC. The assessment focused on:

• Velocity of standard grenades fired with the regular charge (gewöhnliche Ladung)
• Velocity of chilled cast iron grenades using the service charge (Gefechtsladung)
• Maximum achievable velocity without surpassing peak pressure limits of 20 tons per square inch or 3,149 kg/cm²

On 29 January, three standard grenades were fired with 113.4 kg of pebble powder, reaching an average velocity of 470.9 metres per second (1,540 ft/s) at a pressure of 2,190.4 kg/cm². The following day, six chilled cast iron grenades were shot using the service charge of 147.4 kg prismatic powder, attaining an average velocity of 546.9 metres per second (1,790 ft/s) at 2,318.3 kg/cm² pressure. On 6 February, five projectiles were fired with progressively increasing charges; the highest velocity achieved without exceeding maximum pressure was 580.3 metres per second (1,900 ft/s) with a charge of 165.6 kg. This indicated that at 579 metres per second (1,900 ft/s), the gun failed to meet the minimum required velocity of 594 metres per second (1,950 ft/s) while maintaining pressures below 20 tons per square inch, resulting in approximately a 4% reduction in price. Using the service charge, the maximum range was recorded at 7,900 m.

In December 1884, further testing of the 12-inch breechloading guns took place in Cadiz where the weapon was fired at an armored target. During these trials, charges were reduced to simulate firing over longer distances.

Bibliography

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• Inglis, T. (1878). "Targets for the Trial of Recent Battering Ordnance part IV". Professional Papers of the Corps of Royal Engineers. II. The Royal Engineer Institute: 185–202.
• Inglis, T. (1880). "Targets for the Trial of Recent Battering Ordnance". Professional Papers of the Corps of Royal Engineers. IV. The Royal Engineer Institute: 169–194.
• Jones, C. (1885). "Ten and Twelve-inch Armstrong Guns at Cadiz (Excerpt) ". The Engineer. 59–60: 247. {{cite journal}}: no-break space character in |title= at position 54 (help)
• "Her Majesty's ship Conqueror". The Marine Engineer. Whitehall Technical Press: 147–148. 1 October 1880.
• "resumé des Travaux du Comité Supérieur Consultatif de L'Artillerie Espagnole en 1880-1881 (Extrait du Memorial de Artilleria)". Revue d'artillerie. XXI (6). Berguer-Levrault: 537–562. 1883.
• War Office (1886). =19agAAAAMAAJ Treatise on the Manufacture of Guns and Text-book of Service Ordnance. Her Majesty's Stationery Office. p. 240. {{cite book}}: Check |url= value (help)