7.5"/50 (19 cm) Mark III
7.5"/50 (19 cm) Mark IV
Updated 18 February 2009

These guns were mounted on Coastal Defense vessels intended for the Chilean Navy, Constitution and Libertad.  The British took them over in 1906 to prevent their purchase by another European power and renamed them HMS Swiftsure and HMS Triumph, respectively.

The Chileans had chosen this caliber because they were intermediate in size between the standard British mixed secondaries of 6" (15.2 cm) and 9.2" (23.4 cm) guns.  Neither the guns nor the mountings on these ships were identical to each other, a somewhat unusual affair in what were supposedly sister-ships.  This is explained by the fact that the Mark III was built by Elswick while the Mark IV was manufactured by Vickers.

One of the spare Mark III guns for HMS Swiftsure was used to arm the monitor M.26.  In 1917, Swiftsure was decommissioned and two of her guns were then used to arm M.21 and M.23 while the other twelve were used for coast defense in Britain and Belgium.  After HMS Triumph was sunk in May 1915, her two spare Mark IV guns were used to arm the monitors M.24 and M.25.

Mark III was constructed of 'A' tube, breech piece, '1 B' tube and '2 B' tube, wire for 63% of length, 'B' tube and jacket with breech bush screwing into the 'A' tube, breech ring and a ring for connecting the hydraulic buffer to the barrel.  Mark IV was 'A' tube, full length 'B' tube into which the breech bush for the Welin block was screwed, wire for 49% of length, jacket and screwed collar at the rear.  There was a ring for connecting the hydraulic buffer, but no breech ring.

Spare guns were designated as Mark III* and Mark IV*.  The Mark III* differed in having an inner A tube, A tube, wire for 63% of the length, B tube and jacket with breech bush screwing into the A tube, breech ring and "ring connecting hydraulic buffer."  The Mark IV* was constructed of inner A tube, A tube, wire for full barrel length, B tube and jacket.  The breech bush screwed into the A tube.  There was a ring for connecting the hydraulic buffer, but no breech ring.

A total of fourteen Mark III, two Mark III*, fourteen Mark IV and two Mark IV* guns were manufactured.

Nomenclature note:  Despite their designations, these were actually the first 7.5" (19 cm) guns to enter service in the Royal Navy.


HMS Swiftsure in 1908
7.5" (19 cm) guns are in the casemates

Images at The Vickers Photographic Archive

Mark IV:  See photographs 6470, 6796 and 6800

Gun Characteristics
Designation 7.5"/50 (19 cm) Mark III Elswick Pattern A
7.5"/50 (19 cm) Mark IV Vickers Mark B
Ship Class Used On Mark III:  Swiftsure, M21, M23 and M26
Mark IV:  Triumph, M24 and M25
Date Of Design about 1902
Date In Service 1904
Gun Weight Mark III:  15.75 tons (16.0 mt)
Mark IV:  16.06 tons (16.3 mt)
Gun Length oa Mark III:  388.2 in (9.860 m)
Mark IV:  386.7 in (9.822 m)
Bore Length about 375.0 in (9.525 m)
Rifling Length Mark III:  325.2 in (.8.260 m)
Mark IV:  N/A (probably close to Mark III)
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire Battleships:  5 - 6 rounds per minute
Monitors:  4 rounds per minute
Type Bag
Projectile Types and Weights APC - 200 lbs. (90.7 kg)
CPC - 200 lbs. (90.7 kg)
HE - 200 lbs. (90.7 kg)
Bursting Charge APC - N/A
HE - 23 lbs. (10.4 kg)
Projectile Length APC - N/A
HE - 29.7 in (75.4 cm)
Propellant Charge 54.25 lbs. (24.6 kg) MD
Muzzle Velocity 2,827 fps (862 mps)
Working Pressure N/A
Approximate Barrel Life 650 rounds
Ammunition stowage per gun 150 rounds
Note:  Projectiles were 4crh.
Elevation With 200 lbs. (90.7) CPC Shell
Range @  15 degrees 14,328 yards (13,100 m)
Note:  Taken from range tables, but Campbell says that these were probably about 200 yards (180 m) too long.
Armor Penetration with 200 lbs. (90.7 kg) CPC Shell
Range KC Side Armor
10,000 yards (9,140 m) 4.0 in (102 mm)
Data from "Big Gun Monitors."
Range Vertical Steel Armor
3,000 yards (2,700 m) 10 in (254 mm)
Data from "British Battleships 1850 - 1950"
Mount / Turret Data
Designation Single Casemate Mounts
   Swiftsure (14), M21 (1), M23 (1) and M26 (1):  CP Mark III
   Triumph (14), M24 and M25 (1):  CP Mark IV
Weight  32 tons (33 mt)
Elevation -5 / +15 degrees (possibly +14 for main deck guns)
Elevation Rate Manual operation, only
Train about +80 / -80 degrees
Train Rate N/A
Gun recoil N/A
Loading Angle N/A
Note:  Shore mountings were on Arrol platforms and could be wedged up to allow an additional +8 degrees of elevation.
Data from
"Big Gun Monitors:  The History of the Design, Construction and Operation of the Royal Navy's Monitors" by Ian Buxton
"Naval Weapons of World War Two" and "British Naval Guns 1880-1945 No 9" article in "Warship Volume VII" both by John Campbell
"British Battleships:  1860 - 1950" by Oscar Parkes
Special help from Robert Hurst
Page History

 07 January 2007 - Benchmark
25 January 2009 - Fixed minor typographical errors
18 February 2009 - Added construction details