4.5"/45 (11.4 cm) QF Marks I, III and IV
(Marks 2, 3, 4 and 5)
Updated 19 June 2012

These weapons were originally designed to arm the new carriers being built in the 1930s but they eventually became the standard weapon for destroyers built late in World War II and rearmed several capital ships as well.

The Mark I and Mark III guns were used in twin mountings and were interchangeable with each other, the only difference being in details of the firing mechanism.  Several ships actually carried both gun types, depending upon what was available when they were being built or modernized.  The Mark IV was ballistically identical to the Mark I and Mark III guns but slightly modified in order to fit on the standard 4.7" (12 cm) CP XXII single mountings used on destroyers.  The Mark II was an Army AAA weapon, generally similar to the naval guns but not mounted afloat.

Following the failure of the 5.1"/50 (13 cm) QF Mark I, the 4.5" (11.4 cm) caliber was selected in the middle 1930s for a new DP weapon for carriers, as it was believed that this was the largest caliber that could be used for a fixed round.  The complete round for this weapon weighed 85 lbs. (38.6 kg) versus 108 lbs. (49 kg) for the complete round of the 5.1" (13 cm) gun.  However, the decision on the basis of weight was contradictory for two reasons.  First, the slightly lighter 4.7" (12 cm) ammunition for destroyers had always been made in separate form in order to reduce the task of the ammunition handlers.  Second, a fixed round had been designed in the 1920s for the 4.7"/40 (12 cm) Mark XII anti-aircraft gun used on the Nelson class battleships and this weighed 74 lbs. (33.6 kg) complete.  However, during service evaluations, it was found that this weapon could not maintain a high rate of fire - a necessity for an AA weapon - as the heavy round rapidly wore out the gun crews.  So, if by the early 1930s, it was being found that a 74 lbs. (33.6 kg) fixed round was too heavy to allow sustained firing, then it is odd that a few years later an 85 lbs. (38.6 kg) fixed round was deemed to be acceptable.  It would appear that the active Navy and the Admiralty had a considerable lack of communications regarding practical experience with fixed ammunition.

Twin mountings used on carriers and capital ships built before and during World War II fired fixed ammunition.  As noted above, this fixed round proved to be too heavy in service use and there was a tendency for the projectiles to separate from the cartridge cases during normal handling.  These problems resulted in a rapidly decreasing rate of fire during prolonged firing periods.  The mountings for the post-war built carriers Ark Royal and Eagle were modified versions using separate ammunition.

On 8 May 1941 while Renown was engaging Italian torpedo bombers, a fail-safe lockout system malfunctioned and her P3 mounting fired into the back of P2 mounting, killing six and wounding twenty-six.  Angle iron frameworks were later erected around the mountings to prevent a repeat.  The fail-safe gear was overhauled during a refit at Rosyth later that same year.

In 1944, guns of 4.7" (12 cm) caliber, which had been used on nearly every British destroyer built since 1918, gave way to a new destroyer weapon, the 4.5" (11.4 cm) QF Mark IV gun.  It had originally been planned that destroyers would also use fixed ammunition, but reports from the cruisers Scylla and Charybdis told of loading problems during rough weather due to the heavy weight of the ammunition.  As these were 6,000 ton (6,100 mt) cruisers, it was an obvious conclusion that the much lighter destroyers would have even worse handling problems - a conclusion that the Royal Navy had been aware of for over a decade.  To alleviate the problem, a new 4.5" (11.4 cm) gun designed to use separate ammunition was developed for arming destroyers.  Compared to the standard 4.7" (12 cm) Mark IX, this new destroyer weapon had a higher rate of fire and used a heavier projectile with better ballistic properties, but it is questionable if these advantages outweighed the disadvantage of adding yet another mid-caliber weapon and its specialized munitions to the logistical system.  It did have a post-war advantage in that this new caliber became the standard size for Royal Navy destroyers and continues in use to the present day.

Nomenclature note:  In the 1950s the British weapon designation system changed from being per the gun itself to being per the mounting.  At the same time, arabic numerals replaced roman numerals.  Some confusion was created under this new system because older weapons were redesignated, even though the weapons and mountings themselves did not change.  Under this new system, the "4.5-in (11.4 cm) Mark I, Mark III and Mark IV guns" in the "Mark II, Mark III, Mark IV and Mark V mountings" were re-designated as "Mark 2" and "Mark 3" for the twin mounts used on carriers and as "Mark 4" (twin) and "Mark 5" (single) for the mounts used on destroyers.  Asterisks used in the old designation system to denote changes were carried forwards to the new system, but new changes were given "Mod" numbers.  This meant that the Royal Navy now had designations such as "4.5-in Mark 5* Mod 2" which meant that this particular mounting had gone through three modifications since it was first designed.  As could be expected, these changes have led to much confusion as to what weapons were actually used on any particular ship.  For this reason, at the top of this datapage, I show both the original per-the-gun designation and, in parenthesis, the per-the-mounting redesignations.

Gun construction was of an autofretted loose barrel, jacket, removable breech ring and sealing collar.  The breech mechanism used a horizontal sliding breech block with hand-operated closing and semi-automatic opening.  Mark I had both percussion and electric firing while the Mark III was designed for all-electric firing.  The Mark I was later modified to permit electric only firing.  The Mark IV used electric firing and the breech mechanism was considerably lighter with more generous radii on some components.  Five experimental, 46 Mark I, 524 Mark III and at least 199 Mark IV guns were manufactured.  Guns needed to be dismounted in order to change the barrels.  It should be noted that these weapons had a relatively short barrel life.

All British 4.5" naval guns have an actual bore diameter of 4.45" (11.3 cm).


4.5" Mark II BD mountings on HMS Renown
Note the safety rails, which were erected after an accident killed six crewmen and wounded many more
Photograph copyrighted by E.J. Smith (Leading Steward 1940-42)

Click here for additional pictures
Gun Characteristics
Designation 4.5"/45 (11.4 cm) QF Marks I, III and IV
Ship Class Used On Mark I
   Prototype on Iron Duke
   Ark Royal (1937), Forth, Maidstone and Illustrious class
   Queen Elizabeth (3 guns), Valiant (5 guns), Renown (4 guns), Scylla, Charybdis, Tyne, Hecla and Adamant

Mark III
   Queen Elizabeth (7 guns), Valiant (5 guns), Renown (16 guns), Implacable, Savage and Battle classes
   Ark Royal (1950)
   Venezuelan Nueva Esparta class

Mark IV
   Savage, Z, Ca, Ch, Co and Cr classes
   Last eight of the Battle class
   Tribal class (1950s) (Rebuilt Mark V mountings)

Date Of Design about 1935
Date In Service 1938
Gun Weight Marks I and III:  6,304 lbs. (2,859 kg)
Mark IV:  6,179 lbs. (2,803 kg)
Weights include breech mechanism
Gun Length oa 202 in (5.131 m)
Bore Length 200.3 in (5.086 m)
Rifling Length 170.9 in (4.341 m)
Grooves (32) 0.037 in deep x 0.291 (0.94 x 7.39 mm)
Lands 0.1459 in (3.706 mm)
Twist Uniform RH 1 in 25
Chamber Volume 600 in3 (9.83 dm3)
Rate Of Fire Twin mountings of World War II:  12 rounds per minute per gun
Ark Royal (1950) twin mountings:  15 rounds per minute per gun

Destroyer single mountings:  14 rounds per minute
Destroyer twin mountings:  20 rounds per minute per gun

Type Fixed and Separate
Weight of complete round HE - 87 lbs. (39.5 kg)
SAP - 91.75 lbs. (41.6 kg)
Projectile Types and Weights HE - 55 lbs. (24.95 kg)
SAP - 58.25 lbs. (23.0 kg)
Bursting Charge N/A
Projectile Length N/A
Fixed rounds were 49 in (124.5 cm) long
Propellant Charge 11.04 lbs. (5.0 kg) SC 122
13.63 lbs. (6.18 kg) NF/S 198-054
Brass Cartridge -  38.5 lbs. (18.6 kg) with SC charge
Cartridge Case Type, Size and Empty Weight Brass, 114 x 644 mm, about 27.0 lbs. (12.2 kg)
Muzzle Velocity New gun:  2,449 fps (746 mps)
Average gun:  2,350 fps (716 mps)
Working Pressure 20.5 tons/in2 (3,230 kg/cm2)
Approximate Barrel Life 650 rounds
Ammunition stowage per gun
(see Note 5)
Ark Royal (1937), Illustrious and Implacable classes:  400 rounds
Ark Royal (1950):  412 rounds
Indomitable:  385 rounds
Queen Elizabeth, Valiant and Renown:  400 rounds
Battle:  300 rounds
Others:  N/A

1) Outfits were SAP and HE for battleships and aircraft carriers but only HE for depot ships.  By the end of the war, most propellant charges were flashless, if available, and SAP and HE were carried by all except aircraft carriers, which had HE only.  Except in depot ships, 25% of the HE was to have VT proximity fuzes, rising to 50% as availability increased.

2) Destroyers during the war were also equipped with 10 rounds per gun of a RDX/TNT filled HE for use against surfaced submarines.

3) Star shell allowances during the war were 150 to 400 rounds per ship but depot ships had none.

4) Projectiles were 5/10crh.

5) The outfit for Renown per gun was 250 HE rounds, 150 SAP rounds, 75 practice HA rounds and 25 practice LA rounds.  It is believed that this was the same outfit as carried by Queen Elizabeth and Valiant.  The outfit for the early Battle class was 460 SAP, 720 HE, 100 Starshell, 160 LA practice and 160 HA practice.

Elevation With 55 lbs. (25 kg) HE Shell
Range @ 43 degrees 19,900 yards (18,200 m)
Range @ 45 degrees 20,750 yards (18,970 m)
AA Ceiling @ 50 degrees
(Max elevation of single mounting)
29,910 feet (9,120 m)
AA Ceiling @ 80 degrees
(Max elevation of twin mounting)
41,000 feet (12,500 m)
Armor Penetration with 55 lbs. (25 kg) SAP Shell
NC Armor
Deck Armor
10,500 yards (9,600 m)
2.5" (76 mm)
Note:  Data from "British Battleships of World War Two."  Assumes a perpendicular impact.
Mount / Turret Data
(see Note 14)
Single Mounts
   Savage (2), Z (4) and Ca (4) classes:  Mark V
   Ch, Co and Cr classes (4):  RP50 Mark V
   Battle [last 8] class (1):  RP50 Mark V*

Single Mounts - Postwar
   Modernized Ca class (3):  Mark 5* Mod 1
   Tribal class (2):  RP50 Mark 5* Mod 2

Twin Mounts
   Queen Elizabeth (10), Valiant (10), Renown (10) and Illustrious (8) class:  Mark II BD
   Implacable class (8):  RP10 Mark II** BD
   Ark Royal (1950) (8) and Eagle (1950) (8):  Mark II*** BD
   Ark Royal (1937) (8), Scylla (4), Forth (4), Maidstone (4), Tyne (4), Hecla (4) and Adamant (4):  Mark III UD
   Savage (1) and Battle [first 16] (2):  RP10 Mark IV
   Battle [Last 8] (2):  RP10 Mark IV*
   Nueva Esparta (3) class:  N/A (apparently a Mark IV derivative)

Weight  Single Mounts
   Mark V:  11.11 tons (11.29 mt)
   RP50 Mark V:  15.019 tons (15.26 mt)

Twin Mounts
   Mark II BD:  37.95 tons (28.56 mt)
   RP10 Mark II** BD:  43.119 tons (43.811 mt)
   RP10 Mark II*** BD:  49.395 tons (50.188 mt)
   Mark III UD:  29.738 tons (30.215 mt)
   RP10 Mark IV:  45.579 tons (46.310 mt)

Elevation Single Mounts:  -5 / +55 degrees

Twin Mounts:  -5 / +80 degrees

Elevation Rate Mark II:  10 degrees per second
RP10 Mark II**:  20 degrees per second
RP50 Mark V and Mark V*:  20 degrees per second
Others:  N/A (mainly hand-worked)
(see Notes 4 and 6)
All but the below:  about +150 / -150 degrees

Mark IV:  360 degrees
"X" position on ships with Mark V mountings:  360 degrees

Train Rate Mark II:  15 degrees per second
Mark II**:  20 degrees per second
RP50 Mark V and Mark V*:  20 degrees per second
Others:  N/A (mainly hand-worked)
Gun recoil All:  18 in (46 cm)

1) All of the Mark II-type mountings were cramped and difficult to maintain.  These were BD (Between Deck) mountings that had only a low shield above the weather deck with the bulk of the mounting below it.  The original Mark II had a 46 hp motor and oil hydraulic pump in the fixed structure.  The guns were not individually sleeved and run out was by compressed air.  The elevation arcs were driven by pinions on a common shaft and powered by a hydraulic motor.  Training was by hydraulic motor driving a worm gear.  The arrangements for the RP10 Mark II** differed in having a vertical roller race to limit jump and using rack and pinion gears in the power elevation and training rather than worm gears.  The ammunition supply in Ark Royal (1937) was by two-stage electrically driven chain hoists, with the first stage running from the handling rooms to the upper deck and the second stage running from the upper deck to the gun positions.  The Illustrious class had similar dual hoists, but on the upper deck there were horizontal conveyors which carried the shells over to the loading positions for the upper chain hoists.  For the Implacable class, the magazines were located directly under the gun positions and so no horizontal conveyors were needed.  The modernized battleships and battlecruiser used a single stage hoist that brought rounds up into the superstructure.  For all ships, these hoists or conveyors did not come into the gunhouse.  Instead, a human chain was used to carry the rounds from the hoists to a three-round scuttle on each side of the rear of the mounting.  From here, the rounds were passed by hand to a fuze setting machine, one for each gun, and then onto the loading tray.  The loading trays for most Mark II mountings were rotated by hand about a tubular shaft from the cradle and contained the hydraulic and hand rammers.  For the Mark II mountings, the round was grasped by two crewman, one at each end, and together they would lift the round into the loading tray and then push the tray over to behind the breech.  The shell would then load automatically.  The loading trays on the RP10 Mark II** were power-operated.  Ark Royal (1950) was equipped with Inclined Duplex Endless (IDE) chain hoists, one for each mounting.  These hoists ran directly from the magazines to the mountings and could deliver thirty rounds per minute.  Ark Royal (1950) was unique in that she used USN Mark 37 GFCS instead of HACS for gunnery control.

2) Mark II BD, Mark II** BD and Mark III UD mountings used fixed ammunition, all others used separate ammunition.

3) Mark III UD (Upper Deck) mounting was similar to Mark II but had open-backed shields and a smaller roller path diameter of 93 inches (2.36 m) versus 113 inches (2.87 m) for the Mark II.

4) The Mark IV was a true turret mounting that externally resembled the RP10 Mark II** but differed in having a revolving stalk.  The prototype was installed on HMS Savage, where it proved fairly successful.  This mounting-type was then used on the Battle-class destroyers.  The mountings on the Battle class could make one and a half revolutions in each direction from the mid (fore and aft) position for a total of 1080 degrees of train.  The barrels shared a common slide and each had its own ammunition supply.  Each gun had a hydraulic pusher shell and cartridge hoists on the revolving structure and extended one deck down to where they were fed by hand from the fixed electric endless-chain hoists.  Both hoists automatically stopped operating when the "top of the hoist" position was full, but immediately restarted when the ammunition was removed.  Shells were passed via the fuze-setting machines to the loading trays, while the cartridges came up vertically to the rear of the gun.  The loading trays could be charged at any angle.  Rammers were hydraulically operated with a single self-resetting lever control.  A joystick was fitted for local control.  This mounting proved satisfactory in service, but one defect found in the trial mounting on HMS Savage, oil leakage through the decks, was never really solved and continued to plague at least one Battle right up until she finally paid off in 1962.

5) The RP10 Mark IV and RP10 Mark IV* were RPC equipped and had separate shell and cartridge hoists.  The Mark IV* was equipped to use the USA Mark 37 FCS with a Metadyne Mark VII fuze setting machine.

6) Mark V and Mark V* mounts were converted from 4.7" (12 cm) CP XXII mountings.  Besides the differences needed for mounting the 4.5" (11.4 cm) gun, these converted mountings had a shorter recoil distance and were slightly lighter.  Although billed as "dual-purpose," the reality was that these mountings had the same +55 degree maximum elevation of the previous 4.7" (12 cm) guns which had always been seen as too low for defense against aircraft.  These mountings were also hard to work at high elevations, which slowed their rate of fire against aircraft.  The destroyer "X" mounting had modified stops which allowed a total training arc of about 580 degrees.  This is why many photographs of these ships show this mounting stowed pointing dead ahead.

7) The RP50 Mark V mount used electric training and elevation Metadyne RPC equipment.  A joystick was supplied for local control.

8) The single-gun RP50 Mark V* mounting on the last eight Battle class destroyers was equipped to use the USA Mark 37 FCS and had an improved spring-powered loading tray.  Previously, although ramming was spring powered, the tray itself had to be manually pushed into line with the breech and then manually withdrawn after ramming.  The Mark V* had extra springs which allowed the complete loading cycle to be automatically initiated by a single control lever.  The recoil from firing the gun recocked all spring mechanisms.  This mounting was used on the Battle class mainly for firing starshell, but SAP and HE rounds were also supplied.  This mounting also gave these ships an after-firing gun, the lack of which was perceived as being a handicap on the earlier ships of this class.  The Metadyne Mark VII fuze-setters used for these mountings were "not particularly successful."

9) The two Dido class cruisers armed with these guns were originally to have had the 5.25" (13.4 cm) Mark I but production difficulties with these weapons greatly delayed deliveries and the 4.5" (11.4 cm) guns were substituted in their place.

10) When the Ca class destroyers were modernized in the 1950s, the mountings were upgraded with improved RPC equipment and then designated as Mark 5* Mod 1.  At the same time, these ships were also given modern Mark 6M directors linked to Flyplane Mark 5 fire control systems.

11) Single mountings removed from scrapped "C" destroyers were later used on the Tribal class frigates (Type 81) built in the 1950s.  These mountings were then designated as Mark 5* Mod 2.  The most noticeable difference was that the layers' and trainers' sighting ports were plated over to improve the watertightness of the front gun shield.  This mounting was controlled by the MRS3 gunfire control system which was a near-copy of the USN Mark 63 to which it strongly resembles.  It has been noted that the use of these open mounts on modern ASW ships meant that RN sailors would go into battle as exposed to the elements as were their grandfathers in World War I.

12) The gun axes in all twin mountings were 38 in (96.5 cm) apart.

13) Renown had her six forward twin Mark II BD mountings removed in the summer of 1945 after she had been placed into a reduced state.  These mountings were to be reused on carriers, but this was not carried out.  It is speculated that Renown was to get new, more advanced mountings during a refit planned for late 1945, but the end of the war halted all such work and she went to the breakers in 1948.

14) The Mark I mount was a single UD design not used in service.  The RP10 Mark II* BD mount was intended for the Albion class carriers but not used.  Taking the previous note into account, it is possible that these would have been the mountings removed from Renown with some alterations.

Data from
"Nelson to Vanguard:  Warship Design and Development 1923-1945" and "The Design and Construction of British Warships 1939-1945:  Volume I" both by D.K. Brown
"Naval Weapons of World War Two" by John Campbell
"The Naval Institute Guide to World Naval Weapon Systems 1991/92" and "British Carrier Aviation" both by Norman Friedman
"Leander Class Frigate Conversions"  article by Peter Hodges in "Warship Volume III"
"Destroyer Weapons of World War 2" by Peter Hodges and Norman Friedman
"Warships of World War II" by H.T. Lenton and J.J. Colledge
"British Battleships of World War Two" and "British Cruisers of World War Two" both by Alan Raven and John Roberts
"The Battle-cruiser HMS Renown 1916 - 1948" by Peter C. Smith
"Destroyers of World War Two" by M.J. Whitley
 H.M.S. Solebay Association
Page History

29 May 2007 - Benchmark
12 January 2009 - Added cartridge information, added additional information on the Mark II and Mark IV mounts
01 June 2009 - Added ammunition information for Queen Elizabeth, Valiant and Renown, added additional information on shell handling and supply to mounting notes, added note concerning accident to Renown, added picture of mountings on Renown
15 December 2011 - Changed picture from HMS Barfleur to HMS Barrosa
12 February 2012 - Updated to latest template
19 June 2012 - Added mounting information and additional pictures datapage