Gas-checks in British RML heavy guns

Gas-checks were attachments to ammunition that revolutionised the performance of RML heavy guns. The first generation of RML heavy guns began entering service in about 1865. They all had Woolwich rifling and relied on studs on the projectiles for rotation. Gas-checks were first introduced in 1878 or soon after. They significantly reduced wear on the guns while also increasing their range and accuracy. Before long, studless ammunition was being manufactured for these guns, using gas-checks for projectile rotation. Gas-checks also facilitated a switch to the second generation of RML guns which used polygroove rifling and only supported studless ammunition.

The first RML heavy guns were introduced into British service in about 1865. By 1878, 11 models of Woolwich rifled guns had been introduced, ranging from 7 inches to 12.5 inches. Unfortunately, Woolwich rifling had a major defect, namely, that hot powder gas escaping around the ammunition ("Windage") caused excessive barrel erosion, especially in the rifling grooves. Extensive research was performed in the early 1870s to find a solution, and by 1878 two types of gas-checks had been designed. Both gas-checks were made of copper with a little added zinc. They were, in effect, shallow cups of about the same diameter as the ammunition that were attached to the base of the ammunition. When the gun was fired, the powder gas pressure forced the sides of the cup into the rifling grooves, creating a gas seal. It was immediately found that gas-checks also increased the range of guns, and provided greater accuracy. It was also realised that gas-checks were capable of rotating the ammunition, removing the need for studs. This facilitated the use of polygroove rifling, which used a lot of shallow grooves, in place of the Woolwich system which used only a few deeper grooves. Polygroove rifling was less detrimental to the strength of the guns. Also, studless ammunition was stronger and flew more accurately than studded ammunition. Shell strength was particularly important for armour-piercing ammunition.

Attached gas-checks were used with the existing studded ammunition of Woolwich rifled guns. Their basic design was the same across all sizes of guns, the key difference being their diameter. The gas-check was attached to the base of the projectile by a screw-in plug which required a minor modification to each projectile. A few projectiles were unsuitable for modification, and were scrapped.

Fig 1. Attached Gas-Check Mk I.

Fig 2. Attached Gas-Check Mk II.

Fig 1 shows the base of a 9-inch studded Palliser shell with attached gas-check Mk I. The gas-check consists of a wavy disc with a circular collar of the same diameter as the projectile around its edge. On firing, the intense powder gas pressure flattened the disc against the base of the projectile, increasing the disc’s overall diameter, and forcing the collar against and into the rifling, forming a gas seal. The Mk I gas-check performed well for the faster burning R.L.G. powder with which it was initially tested, but was found to seal the grooves too slowly with slower burning powders such as P and P², resulting in scoring of the bottoms of grooves. As a result, the Mk I gas-check was superseded by the Mk II gas-check by 1881. Fig 2 shows a 9-inch studded common shell with a Mk II gas-check. The Mk II gas-check was virtually identical to the Mk I, the main difference being that the collar had projections that fitted into the grooves on loading and resulted in faster gas sealing on firing.

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