Armstrong gun

An Armstrong gun was a type of rifled breech-loading field and heavy artillery piece designed by Sir William Armstrong. It was first manufactured in England starting in 1855 by the Elswick Ordnance Company and the Royal Arsenal at Woolwich. The Armstrong gun employed a distinctive built-up gun construction method. The core of the gun consisted of a wrought iron (and later, mild steel) inner tube, which was reinforced by a series of wrought-iron coils shrunk over it. This design kept the inner tube under constant compression, increasing its strength and resistance to internal pressure generated during firing.

In 1854, Armstrong approached the Secretary of State for War with a proposal to construct a rifled breech-loading 3-pounder gun for evaluation. The gun was later modified to a 5-pounder, and the design proved successful in terms of both range and accuracy. Over the next three years, Armstrong further developed his system of construction and adapted it to guns of larger calibres.

Armstrong’s system was officially adopted in 1858, initially for “special service in the field.” At first, production was limited to smaller artillery pieces, including 6-pounder (2.5 in / 64 mm) mountain or light field guns, 9-pounder (3 in / 76 mm) guns for horse artillery, and 12-pounder (3 in / 76 mm) field guns.

Although Armstrong did not consider his system suitable for heavier guns, the government directed him to develop larger calibres, including the 20-pounder (3.75 in / 95 mm) field and naval gun, the 40-pounder (4.75 inches (121 mm)) siege gun, and the 110-pounder (7 in / 180 mm) heavy gun. The Royal Navy adopted all three guns, and all except the 20-pounder saw service in New Zealand.

Armstrong’s guns used a “built-up” construction, comprising a central “A” tube—initially made of wrought iron, and from 1863 onwards of mild steel toughened by oil treatment—forming the bore. Over this tube were shrunk several wrought-iron coils which kept the central tube under compression, along with a breech-piece and a trunnion ring. The guns employed a “polygroove” rifling system: the bore had 38 grooves along its length, with a twist rate of one complete turn per 38 calibres.

The cast iron shell, shaped similarly to a Minié ball, had a thin lead coating making it slightly larger than the bore diameter; this coating engaged with the rifling grooves to impart spin. This system was originally developed by Martin von Wahrendorff and Giovanni Cavalli in Sweden. The spin stabilization, combined with the elimination of windage due to the tight fit, allowed the gun to achieve greater range and accuracy than contemporary smoothbore muzzle-loaders while using a smaller powder charge.

Each gunpowder cartridge was topped with a “lubricator” composed of a mixture of tallow and linseed oil contained between two tin plates, backed by a felt wad coated with beeswax, and finally sealed with millboard. The lubricator followed the shell down the bore; the lubricant was squeezed out between the tin plates, while the wad cleaned lead deposits left by the shell’s coating, leaving the bore clean for subsequent rounds.

A particularly innovative feature, more commonly associated with 20th-century artillery, was Armstrong’s “grip”: a slight reduction in diameter over the last 6 inches of the bore at the muzzle end, effectively a squeeze bore. This section centred the shell before leaving the barrel and swaged down its lead coating slightly, reducing its diameter and improving ballistic performance.