RBL 7-inch Armstrong gun

Ordnance RBL 7 inch gun
A gun on a wooden slide carriage in the 19th century.
Type	Naval gun Coast Defence gun
Place of origin	United Kingdom
Service history
In service	1861 - 190?
Used by	United Kingdom
Wars	New Zealand Land Wars Bombardment of Kagoshima Bombardment of Shimonoseki
Production history
Designer	W.G. Armstrong Co.
Manufacturer	W.G. Armstrong Co. Royal Gun Factory
Unit cost	£425 - £650 (£47000-£72000 in 2013)
Produced	1859 - 1864
No. built	959
Variants	82cwt, 72cwt
Specifications
Barrel length	99.5 inches (2.527 m) bore (14.21 calibres)
Shell	90 to 109 pounds (40.82 to 49.44 kg)
Calibre	7-inch (177.8 mm)
Breech	Armstrong screw with vertical sliding vent-piece (block)
Muzzle velocity	1,100 feet per second (340 m/s)
Maximum firing range	3,500 yards (3,200 m)

The Armstrong RBL 7 inch gun, also known as the 110-pounder, was an early attempt to use William Armstrong's new and innovative rifled breechloading mechanism for heavy rifled guns.

The Armstrong "screw" breech mechanism used a heavy block inserted in a vertical slot in the barrel behind the chamber, with a large hollow screw behind it which was manually screwed tight against the block after loading. A metal cup on the front of the block, together with the pressure of the screw behind it, provided "obturation" and sealed the breech to prevent escape of gasses rearward on firing. The sliding block was known as the "vent-piece", as the vent tube was inserted through it to fire the gun. In modern terms it was a vertical sliding block.

To load the gun, the vent-piece was raised, the shell was inserted through the hollow screw and rammed home into the bore, and the powder cartridge was likewise inserted through the screw into the chamber. The vent-piece was lowered, the screw was tightened, a tube was inserted in the top of the vent-piece, and the gun was fired.

Shells had a thin lead coating which made them fractionally larger than the gun's bore, and which engaged with the gun's rifling grooves to impart spin to the shell. This spin, together with the elimination of windage as a result of the tight fit, enabled the gun to achieve greater range and accuracy than existing smoothbore muzzle-loaders with a smaller powder charge.

On top of each powder cartridge was a "lubricator" consisting of tallow and linseed oil between two tin plates, backed by a felt wad coated with beeswax and finally by millboard. The lubricator followed the shell down the bore, the lubricant was squeezed out between the tin plates and the wad behind it cleaned out any lead deposits left from the shell coating, leaving the bore clean for the next round.

This method had already proved successful in the much smaller RBL 12 pounder 8 cwt field gun, and the British Government requested it be implemented for heavy guns despite Armstrong's protests that the mechanism was unsuited to heavy guns :

"The threatening aspects of the continent required that large rifled guns should be procured for naval and siege purposes. I was therefore called upon to produce 40-pounders and 100-pounders without having had an opportunity of testing the patterns by previous trials, though I had stated in my original report that I apprehended that the application of breech-loading to large guns would involve an application of parts which would be inconveniently heavy to handle... I was at first in hopes that the same material which had been used and found to be sufficient for the 40-pounder, would be found equally suitable for the 100-pounder; but that turns out not to be the case. The vent-piece for the 100-pounder continues still to be a difficulty". Sir W Armstrong to the Select Committee on Ordnance in 1863.