The 7 in (18 cm) referred to, and introduced into the service in 1865, were the first of the so-called Woolwich guns, which then meant "wrought iron M.L.

guns built up on Sir W. Armstrong's principle, improved upon by hooking the coils over one another, and having solid ended steel barrels, rifled on the system shown above, for studded projectiles".

[4] All versions were constructed of a steel A tube surrounded by various numbers and thicknesses of wrought-iron coils.

This was a coast defence gun introduced in 1865 to replace the failed RBL 7-inch Armstrong gun.Three marks were produced.

Early models were made by simply turning off some of the jacket around 7 in (18 cm) 6½ ton guns, as firing with reduced charges placed less strain on the coils.