To counter the perceived threat from the Italians, Inflexible was to be equipped with four of the largest guns available, weighing 60 tons each.
In October 1874 it was decided to modify the design of Inflexible to use an even bigger gun which Armstrongs was producing, a 16-inch (406 mm) gun weighing 81 tons.
The Italians responded by changing their design to take even larger 100-ton 17.7-inch (450 mm) Armstrongs guns.
As these could not be fitted to Inflexible, four examples were ordered by the British Government, two each for the coastal defences around Gibraltar and Malta respectively.
Two of these guns still exist, at Fort Rinnella on Malta and at the Napier of Magdala Battery on Gibraltar.

The four 81-ton muzzle-loading rifles were mounted in two 33-foot-10-inch-diameter (10.31 m) turrets mounted en echelon, with the forward turret mounted on the port side of the ship and the after turret on the starboard side.
The superstructure both fore and aft was very narrow to allow one gun in each turret to fire axially, i.e. directly forward or directly aft.
In practice, as in previous ships, it was found that axial fire led to so much blast damage to the ship's superstructure that it was impractical.
However, the en-echelon arrangement also meant that at least three guns could fire on bearings close to fore and aft.
All four guns could be fired broadside.

The en-echelon configuration was retained for the two ships of the Colossus class, but subsequently abandoned in the Royal Navy in favour of centreline mounts at either end of the ship.
The en-echelon configuration did not reappear in Royal Navy capital ships until HMS Neptune launched in 1909.

Each turret weighed 750 tons and was protected by an outer layer of 9 inches (230 mm) of compound armour, an inner layer of 7-inch-thick (180 mm) wrought iron, with a total of 18 inches (460 mm) of teak backing.
The turrets were rotated hydraulically, taking around a minute to perform a complete rotation.

Inflexible's guns were muzzle loaded, and because of their length could not be reloaded from inside the turrets.
Consequently reloading was done using hydraulic rams fitted outside the two turrets underneath an armoured glacis.
To reload the guns, the turret was rotated to align the guns with the rams, and the guns depressed so that the rams could push the gunpowder charge and 1,684-pound shell into it.
The rams had to be extended twice: First, to extinguish any burning material remaining inside the gun using a sponge and water jet fixed to the end of the ram, and then again after charge, shell and wadding had been placed on a loading tray in front of it to be driven into the gun.
The shell had a copper disk at its base which engaged with rifled grooves cut into the barrel to spin the shell, rather than zinc studs used on earlier designs.
Tests showed that the normal full charge of 450 pounds of brown prismatic gunpowder would produce a muzzle velocity of 1,590 feet per second (480 m/s), which could penetrate 23 inches (580 mm) of wrought iron armour at 1,000 yards (910 m).
The muzzle loading took between 2.5 and four minutes.
To counter the perceived threat from the Italians, Inflexible was to be equipped with four of the largest guns available, weighing 60 tons each. In October 1874 it was decided to modify the design of Inflexible to use an even bigger gun which Armstrongs was producing, a 16-inch (406 mm) gun weighing 81 tons. The Italians responded by changing their design to take even larger 100-ton 17.7-inch (450 mm) Armstrongs guns. As these could not be fitted to Inflexible, four examples were ordered by the British Government, two each for the coastal defences around Gibraltar and Malta respectively. Two of these guns still exist, at Fort Rinnella on Malta and at the Napier of Magdala Battery on Gibraltar.
The four 81-ton muzzle-loading rifles were mounted in two 33-foot-10-inch-diameter (10.31 m) turrets mounted en echelon, with the forward turret mounted on the port side of the ship and the after turret on the starboard side. The superstructure both fore and aft was very narrow to allow one gun in each turret to fire axially, i.e. directly forward or directly aft. In practice, as in previous ships, it was found that axial fire led to so much blast damage to the ship's superstructure that it was impractical. However, the en-echelon arrangement also meant that at least three guns could fire on bearings close to fore and aft. All four guns could be fired broadside.
The en-echelon configuration was retained for the two ships of the Colossus class, but subsequently abandoned in the Royal Navy in favour of centreline mounts at either end of the ship. The en-echelon configuration did not reappear in Royal Navy capital ships until HMS Neptune launched in 1909.
Each turret weighed 750 tons and was protected by an outer layer of 9 inches (230 mm) of compound armour, an inner layer of 7-inch-thick (180 mm) wrought iron, with a total of 18 inches (460 mm) of teak backing. The turrets were rotated hydraulically, taking around a minute to perform a complete rotation.
Inflexible's guns were muzzle loaded, and because of their length could not be reloaded from inside the turrets. Consequently reloading was done using hydraulic rams fitted outside the two turrets underneath an armoured glacis. To reload the guns, the turret was rotated to align the guns with the rams, and the guns depressed so that the rams could push the gunpowder charge and 1,684-pound shell into it. The rams had to be extended twice: First, to extinguish any burning material remaining inside the gun using a sponge and water jet fixed to the end of the ram, and then again after charge, shell and wadding had been placed on a loading tray in front of it to be driven into the gun. The shell had a copper disk at its base which engaged with rifled grooves cut into the barrel to spin the shell, rather than zinc studs used on earlier designs. Tests showed that the normal full charge of 450 pounds of brown prismatic gunpowder would produce a muzzle velocity of 1,590 feet per second (480 m/s), which could penetrate 23 inches (580 mm) of wrought iron armour at 1,000 yards (910 m). The muzzle loading took between 2.5 and four minutes.
HMS Inflexible (1876)