Description
The Armstrong "screw" breech mechanism used involved 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.
Read more about this topic: RBL 7 Inch Armstrong Gun
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