This article explains terms used for the British Armed Forces' ordnance (i.e.: weapons) and also ammunition. The terms may have slightly different meanings in the military of other countries.
Between decks : applies to a naval gun mounting in which part of the rotating mass is below the deck, and part of it is above the deck. This allows for a lower profile of turret, meaning that turrets need not be superfiring (i.e. they can be mounted on the same deck and not obstruct each other at high angles of elevation.)
The term BL, in its general sense, stood for breech loading, and contrasted with muzzle loading. The shell was loaded via the breech (i.e. the gunner's end of the barrel, which opened) followed by the propellant charge, and the breech mechanism was closed to seal the chamber.
Breech loading, in its formal British ordnance sense, served to identify the gun as the type of rifled breechloading gun for which the powder charge was loaded in a silk or cloth bag and the breech mechanism was responsible for "obturation" i.e. sealing the chamber to prevent escape of the propellant gases.〔(Royal New Zealand Artillery Old Comrades Association, Breech Mechanisms )〕 The term BL was first used to refer to the Armstrong breechloaders, introduced in 1859. Following the discontinuation of Armstrong breechloaders and the period of British rifled muzzle-loaders (RML), British breechloaders were re-introduced in 1880. At this point the term RBL was retrospectively introduced to refer to the Armstrong breechloaders, which had a totally different breech mechanism, and since then the term BL has applied exclusively to the type of breechloader introduced from 1880 onwards using interrupted-screw breeches.
Early British Elswick breechloaders in the 1880s used a steel "cup" obturation method. This was quickly superseded in guns designed by the Royal Gun Factory by the French de Bange method, the basic principle of which is still in use today. In British service this became a Crossley pad with an interrupted thread screw block e.g. a Welin screw. The shell was loaded via the breech, followed by the propellant charge in a cloth bag. A single-use "vent sealing tube", a type of primer not dissimilar in appearance to a blank rifle round, was inserted into the breech for firing the gun.
The term "BL" contrasts with QF guns, for which the propellant charge was loaded enclosed in a brass cartridge case which expanded on firing and sealed the breech. For instance, Britain before World War I had both QF and BL 6 inch guns. Both were "breech loading" in the general sense, but in the formal nomenclature it separated 6 inch guns with breeches designed for charges in brass cartridge cases (QF) from those designed for cloth bag charges (BL).
Shells designed for one type were not necessarily suitable for use in the other type; for instance, a BL shell relied upon the tight fit of its driving band in the gun bore to prevent it slipping back when the gun was elevated, but a QF shell could rely upon the cartridge case, either fixed or separate, to prevent it slipping back. This presented difficulties for BL guns at high angles. A special cartridge was developed for BL 9.2 inch guns on H.A. mountings, with provision for a wooden (beech) stick to be inserted through the centre to prevent the shell slipping back on elevation.〔Treatise on Ammunition 10th Edition 1915, Page 77〕
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