- A.N. Other and NHSA Webmaster
- Naval technology
- RAN Ships
- None noted.
- September 1981 edition of the Naval Historical Review (all rights reserved)
AFTER THE UNSATISFACTORY PERFORMANCE of the Armstrong screw breech, a retrograde step was taken in returning to the Rifled Muzzle Loading system for the guns of both sea and land service. Design work continued with breech loaders, at a very slow pace. On the continent breech loading systems were in use and were working. Two distinct types of breeches had been developed, the wedge breech using a brass cartridge case, and the interrupted screw using a bagged charge of the same type used by Armstrongs. This article deals with the latter.
Historians are divided as to who should get credit for the interrupted screw but usually give the French all the kudos. Practically all breeches of this type have been developed from the original patent taken out by an American named Ben Chambers, who patented a slotted screw breech in 1849. This type was eventually acquired by General de Beaulieu of the French Army, and further development in France saw the system in efficient form. Evidently Chambers’ original design was extremely crude, but the idea was there.
In the interrupted screw breech threads are cut in the breech end of the barrel, some sections of the thread being machined away. A screwed plug, known in the British service as the breech block, is formed to fit into the barrel, and it also has sections of the threads cut away. To close the breech the block is entered with the threaded portions lining up with the unthreaded portion in the barrel. A turn in one direction, usually to the right, locks the threaded sections and effectively closes the breech.
To allow the system to work, a ring carrier had to be hinged to the barrel, this carrier allowing the breech block to be withdrawn or inserted. Thus three distinct movements were required to open or close the breech.
First motion was to turn the block to unlock. Second motion was to withdraw the block straight back through the ring carrier. The third movement was to swing the block clear of the barrel for loading. Breeches of this type are referred to as Three Motion Breeches. Sealing the escape of the gasses during firing was handled in a couple of different ways. The French used the De Bange system where a mushroom headed bolt worked in a hole bored through the breech block. Between the rear face of the bolt and the front face of the breech block was an expanding pad. On firing, the force of the burning of the charge forced the bolt back, squeezing the expanding pad until it sealed the chamber. The mushroom headed bolt had a vent hole drilled through it to allow the ignition flame to fire the propellant charge, and the bolt became known as the Axial Vent.
In the British service the first type of interrupted screw breech used a system known as the Elswick Cup. In this system a tin cup was fitted on the face of the breech block, and this was expected to expand into the chamber and so seal the breech. It was only moderately successful, and was not used for any great time, even though quite a few of the earlier types were so fitted.
Steps were next taken to improve the old Three Motion Breech, and the Continuous Motion was adopted. This was a step in the right direction, but the system was still slow in operation. Finally a Single Motion Breech was designed, all motions being controlled by a single Breech Mechanism Lever, known to all gunners as the BM Lever. With this type the old ring carrier was done away with and this system has stood the test of time.
Firing the charge was accomplished by many methods, but finally ended up with either electric or percussion primers being used. To fire the charge, a fitting known as the firing lock was attached to the rear of the breech, and into this lock a tube was placed. On either the closing of an electric circuit or the blow from a hammer in the case of a percussion tube, a flame went through the axial vent and fired the charge.
Loading and firing a heavy gun of this type was quite an operation, considering that the original 6 inch breech loaders fired a shell weighing 80 pounds, increased to 112 pounds in the later models. To help the loading number get the shell into the breech a shot guide was provided, the guide swinging into position behind the breech as the block swung clear. The projectile was then entered into the chamber and rammed home. Next the charge was pushed in behind the projectile as the breech closed.