Adopted in 1865. There are three patterns of the 6½-ton guns, the first being of the old Armstrong construction, recognizable by its soda-water bottle shape. The other two marks are of the improved construction. The 90-cwt. gun was adopted in 1874 for use in the armament of unarmored vessels. It is similar to the Mark III. 6½-ton gun, except that it is lighter, the outside being turned down to get the required lightness. These guns are very similar in build and exterior appearance to the 64-pdr. _8-inch R. M. L. 9 Tons._ Introduced in 1866. There are three marks of this gun, similar to the 7-inch patterns. The only way in which the 8-inch can be distinguished from the 7-inch without measuring the diameter of the bore is by the fact of its having four grooves in place of three. _9-inch R. M. L. 12 Tons._ Introduced in 1865. There are five marks of this gun, the first one being of the old Armstrong construction, and the others of the Woolwich developments. The distinctive mark of this calibre is the grooving, there being six grooves. _10-inch R. M. L. 18 Tons._ Introduced in 1868. There are two patterns of this gun, both of the Woolwich design. The distinguishing marks are the nine grooves. _11-inch R. M. L. 25 Tons._ Introduced in 1867. There are two patterns of this calibre, both Woolwich. But few of them are used in the navy. This calibre cannot be distinguished from the 12-inch, except by measurement. _12-inch R. M. L. 25 Tons and 35 Tons._ The 25-ton gun was introduced in 1864. There are two patterns, the first built on the old Armstrong pattern, there being but four in service. The second mark is Woolwich. The 35-ton gun was introduced in 1871. There is but one mark of this gun corresponding to the Woolwich. [Illustration: 12-inch Mark I.] _12½-inch R. M. L. 38 Tons._ Introduced in 1875. There is but one mark of this calibre, and it exactly resembles the 35-ton 12-inch. _16-inch R. M. L. 80 Tons._ Introduced in 1878. There is but one mark of this calibre. These guns have a chamber one inch greater in diameter than the bore, and the form of rifling is that used with expanding projectiles, being multigroove with shallow grooves. The construction corresponds to the latest development of the Woolwich pattern. [Illustration: 16-inch.] _Armstrong Construction._ The first type of rifled guns introduced into the English naval service was the Armstrong, for both breech and muzzle loaders. Fabrication of original breech-loader ceased in 1864, but many of this type still remain in service. The original construction of the muzzle-loader was modified in 1864, but the general type and principles were preserved and are still used. The present type of modified Armstrong construction is used by many foreign services, and the 100-ton gun has been introduced in the English service. The modified Armstrong breech-loading system for the 100-ton gun has been adopted in Italy.