These guns were the first to incorporate the new "Woolwich" rifling system, a modification of the French system, of from 3 – 9 broad shallow grooves after Britain abandoned the Armstrong "shunt" rifling system in May 1865 : "...M.L. 7-inch guns in course of manufacture were rifled on this principle, upon which all of our heavy pieces since have been rifled.
The 7-inch 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".

All versions were constructed of a steel A tube surrounded by various numbers and thicknesses of wrought-iron coils.
Rifling was 3 grooves with a uniform 1 turn in 35 calibres i.e. in 245 inches.

The diagrams below show the progression from the original expensive Armstrong construction in Mk I of multiple relatively thin coils, through to the simplified and cheaper Woolwich design of Mk III.
These guns were the first to incorporate the new "Woolwich" rifling system, a modification of the French system, of from 3 – 9 broad shallow grooves after Britain abandoned the Armstrong "shunt" rifling system in May 1865 : "...M.L. 7-inch guns in course of manufacture were rifled on this principle, upon which all of our heavy pieces since have been rifled. The 7-inch 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".
All versions were constructed of a steel A tube surrounded by various numbers and thicknesses of wrought-iron coils. Rifling was 3 grooves with a uniform 1 turn in 35 calibres i.e. in 245 inches.
The diagrams below show the progression from the original expensive Armstrong construction in Mk I of multiple relatively thin coils, through to the simplified and cheaper Woolwich design of Mk III.
RML 7-inch gun