The first experimental 600pr RML of 25tons had a calibre of 12 inches and was tested at Shoeburyness in February 1858. Although a number of the 25ton guns were made for the Navy in the 1860s i.e. for HMS Monarch and Captain by 1870 a heavier gun was proposed. By October 1871 it was re-bored to 12-inch with a weight of 35tons. By 1872 this gun, with an increased bore length and a weight of 36tons, was issued for land service. In 1873 four of the 12-inch 35ton RMLs were mounted in Devastation. In the following year the original 11.6/12-inch had been re-tubed, weighed 38 tons and was then bored out to 12.5-inches. The production of this gun commenced in 1875 with 21 built in 1875/76. Most were deployed for Coast defence although Dreadnought had four of them.
The heaviest RML trialled at Shoeburyness, in 1876, was the 16-inch of 80tons, resulting in two of being mounted in the Admiralty Pier, Dover. Then of course came the 17.72-inch of 100tons for Malta and Gibraltar!