The Armstrong RBL 20lber was a breech loading Field & Naval gun developed in the late 1850's for the British Armed Forces. The RBL 20lber was among the first breech loading artillery pieces to be used by any part of the British Armed Forces.
The Armstrong RBL 20lber was made entirely from cast iron, with a 84in (2.1m) barrel and a rather unusual (for the time) screw with a vertically sliding vent to block the breech after the shell was loaded. The Navy version of the RBL 20lber was identical to the Field version, except that around 30in of barrel was removed, lightening the gun so it could be better moved around smaller ships. In both formats the barrel was given rifling, which was fast becoming a growing practice in firearm manufacture.
The RBL 20lber was mounted on the familiar wheeled carriage with a box trail to support the gun. However, as the RBL 20lber fired a powerful shell (21.8lb (9.9kg) in weight), the overall weight of the RBL 20lber came in at 1,785lb (810kg) or, in British Artillery terms, at 16cwt (16-hundredweight). Even with the shortened barrel of the Naval version the RBL 20lber weighed in at between 1,455lb (660kg) and 1,675lb (760kg).
The Armstrong RBL 20lber was designed to fire a 21.8lb (9.9kg) shell, normally used aboard smaller ships in the Royal Navy, such as sloops. The Naval version, due to the shorter barrel, would achieve a slower muzzle velocity compared to the Field version, around 1,000ft/s (300m/s) compared to the 1,130ft/s (340m/s) of the Field gun version.
The Armstrong RBL 20lber was first adopted in 1859, mainly fitted to lighter ships such as sloops, as eluded to earlier. It was intended to be used for broadsides, whereby all guns on one side of a ship fired simultaneously against a close target, although there is little known use of the RBL 20lber before it was replaced.
Similarly the heavier Field gun version of the RBL 20lber has little known about its use. The Armstrong RBL 20lber remained in use until the 20th century (the official date is unknown) with 412 examples constructed.