When Britain adopted rifled ordnance in the 1860s it still had large stocks of serviceable but now obsolete smoothbore guns.
Gun barrels were expensive to manufacture, so the best and most recent models were selected for conversion to rifled guns, for use as second-line ordnance, using a technique designed by William Palliser.
The Palliser conversion was based on what was accepted as a sound principle that the strongest material in the barrel construction should be innermost, and hence a new tube of stronger wrought iron was inserted in the old cast iron barrel, rather than attempting to reinforce the old barrel from the outside.

This gun was based on the cast-iron barrel of the Millar Pattern 8-inch 65 cwt gun, originally designed in 1834.
This was designed to fire a smooth bore spherical shell weighing 50 pounds.
The 8-inch gun was bored out to 10.5 inches and a new built-up wrought iron inner tube with inner diameter of 6.29 inches was inserted and fastened in place.
The gun was then rifled with 3 grooves, with a uniform twist of 1 turn in 40 calibres (i.e. 1 turn in 252 inches), and proof fired.
The proof firing also served to expand the new tube slightly and ensure a tight fit in the old iron tube.
When Britain adopted rifled ordnance in the 1860s it still had large stocks of serviceable but now obsolete smoothbore guns. Gun barrels were expensive to manufacture, so the best and most recent models were selected for conversion to rifled guns, for use as second-line ordnance, using a technique designed by William Palliser. The Palliser conversion was based on what was accepted as a sound principle that the strongest material in the barrel construction should be innermost, and hence a new tube of stronger wrought iron was inserted in the old cast iron barrel, rather than attempting to reinforce the old barrel from the outside.
This gun was based on the cast-iron barrel of the Millar Pattern 8-inch 65 cwt gun, originally designed in 1834. This was designed to fire a smooth bore spherical shell weighing 50 pounds. The 8-inch gun was bored out to 10.5 inches and a new built-up wrought iron inner tube with inner diameter of 6.29 inches was inserted and fastened in place. The gun was then rifled with 3 grooves, with a uniform twist of 1 turn in 40 calibres (i.e. 1 turn in 252 inches), and proof fired. The proof firing also served to expand the new tube slightly and ensure a tight fit in the old iron tube.
RML 64-pounder 71 cwt gun