RBL
"Rifled Breech Loading". Refers to the first generation of British rifled breech loading guns introduced in 1859 which used the unique Armstrong "screw breech" and included the RBL 12 pounder field gun and RBL 7 inch naval gun. These guns were originally known as "BL" (breech loading); the term "RBL" was introduced retrospectively in the 1880s to differentiate these Armstrong designs from the second unrelated generation of rifled breech loaders beginning in 1880 which are referred to as BL. The "RBL" guns were considered to be failures and Britain reverted to RML (rifled muzzle-loading) guns from the mid-1860s to 1880.
Read more about this topic: List Of British Ordnance Terms