The Gun - Rifled Ordnance: Heavy Field and Siege Artillery
OrdnanceAs already mentioned, after Britain's decision to revert from RBL to RML ordnance, the production of heavy Naval and coast guns took priority, commencing during the late 1860s. The introduction of RML field guns did not take place until the 1870s. No conversions of RBL guns to RML were made.
The smallest of the class was the RML 25-pr 18-cwt (4-in) gun, proposed in 1871 as a 'gun of position', eg coast artillery, and as a light siege piece. However, the decision to introduce it into the service was not made until 1874.
In construction the 25-pr resembled the RML 16-pr mentioned below. It was rifled according to the Woolwich system, three grooves with a uniform twist of one turn in 35 calibres.
Two marks of this gun were made. The RML 40-pr 34-cwt Mark 1 (4.75-inch) was intended to replace the RML 40-pr (4.75-inch) but did not shoot consistently owing to incomplete combustion of the propellant charge. It was therefore replaced in 1874 by a Mark 2 version of the 35-cwt and some 20 inches longer.
RML 6.6-inchIn 1876 in an attempt to increase the power of the 64-pr 64-cwt (6.3-in) gun, a number were converted to 6.6-inch. The conversion consisted in boring out the "A" tube to the full depth of the grooves (0.3-inch) to a diameter of 6.6 inches, and re-rifling it with a polygroove system of 20 grooves with a twist increasing from one turn in 100 calibres at the breech to one in 35 at 13.2 inches from the muzzle.
The strength of the gun was not reduced to any extent for the reduction in thickness was balanced by the advantage of the shallow polygroove rifling which distributed the stresses caused by firing more evenly. The calibre of 6.6 inches was also selected to enable the same projectile to be fired in the 6.6-inch howitzer, manufacture of which was about to begin.
Those 64-pr guns converted were not successful so new pieces of 6.6 inch calibre were manufactured. Although primarily intended for service in the siege train some of the 6.6-inch guns found their way into forts. Projectile weight (shell) was 100 lbs, MV with full charge of 25 lbs gunpowder 1410 fs.
CarriagesOf iron and steel, these were heavier versions of field types. The RML 25-pr was mounted on the same carriage as the RML 16-pr which in turn was slightly heavier than that for the RML 13-pr. Left: Carriage for the RML 40-pr gun and 6.3- and 6.6-in howitzers. The piece shown mounted in the drawing is a BL 5-in gun for which the carriage was later modified with the advent of BL equipments.