RBL 40 pounder Armstrong gun

Ordnance RBL 40 pounder gun
Diagram from 'Treatise on Service Ordnance' (HMSO), 1877
Type	Naval gun Fortification gun
Place of origin	United Kingdom
Service history
In service	1860s - 1900?
Used by	United Kingdom Australian colonies
Wars	New Zealand Land Wars Bombardment of Kagoshima
Production history
Designer	W.G. Armstrong Co.
Manufacturer	W.G. Armstrong Co. Royal Gun Factory
Produced	1859 - 1863
No. built	1013
Variants	32cwt, 35cwt
Specifications
Weight	32 cwt (3,584 pounds (1,626 kg)), later 35 cwt (3,920 pounds (1,780 kg)) gun & breech
Barrel length	106.3 inches (2.700 m) bore & chamber
Shell	40 pounds 2 ounces (18.20 kg)
Calibre	4.75-inch (120.6 mm)
Breech	Armstrong screw with vertical sliding vent-piece (block)
Muzzle velocity	1,180 feet per second (360 m/s)

The Armstrong RBL 40 pounder gun was introduced into use in 1860 for service on both land and sea. It used William Armstrong's new and innovative rifled breechloading mechanism. It remained in use until 1902 when replaced by more modern Breech Loading (BL) guns.

The Armstrong "screw" breech had already proved successful in the RBL 12 pounder 8 cwt field gun, and the British Government requested it be implemented for heavier guns despite Armstrong's protests that the mechanism was unsuited to heavy guns. Guns were produced at both the Royal Gun Factory in Woolwich, and the Elswick Ordnance Company.

Like other early Armstrong guns they were rifled on a polygroove system, firing a variety of lead coated projectiles.

The first version weighed 32 cwt, followed by the 35 cwt version which introduced a longer and stronger breech-piece. A 32 cwt variant having a horizontal sliding-wedge breech instead of the Armstrong screw with vertical vent-piece was introduced in 1864 as an attempt to address the perceived weaknesses of the screw-breech design. It was withdrawn from service by 1877.

From 1880 a small number of 35 cwt guns had their trunnion rings rotated to the left to allow the vent-piece to open horizontally to the right, being known as "side-closing" guns. They differed from the wedge guns in that the vent piece was still locked in place by tightening the screw behind it.

The gun was recommended in 1859 for the Navy as a broadside or pivot gun.

An officer from HMS Euryalus described the gun's performance at the Bombardment of Kagoshima of August 1863:

Following the bombardment of Alexandria in 1882, as part of the Anglo-Egyptian War, an armed train was employed. One 40 Pounder RBL was mounted onto the train and manned by men of the Royal Navy. It saw some action at the battle of Kassasin on 1 September 1882.

A number of different carriages for guns employed for Land Service were available. A wooden siege carriage with wheels and attached limbers, enabled the guns to be drawn by teams of heavy horses.