These guns had their genesis in the Kamikaze attacks of World War II. A larger-caliber projectile was needed to replace the 20 mm Oerlikon and 40 mm Bofors automatic weapons as they had proven to be too small to kill-stop suicide planes. This need led to the accelerated development of 3"/50 (7.62 cm) guns equipped with autoloaders and later to the 3"/70 (7.62 cm) project. The 3" (7.62 cm) caliber was selected for these weapons as it represented the smallest-size projectile that could still be fitted with a VT (radar proximity) fuze. BuOrd considered the 3"/70 (7.62 cm) twin mounting to be superior to the single 5"/54 (12.7 cm) Mark 42 for defending against fast aircraft and guided missiles.
The development of the ammunition and water-cooled barrel was a joint British-American project, but each navy designed a completely different gun mounting, with the British one being the 3"/70 (7.62 cm) Mark VI.
This twin mount was intended to replace the older 5"/38 (12.7 cm) Mark 12 mounts on a one-for-one basis and to become the standard weapon for post-war destroyers. However, its complex nature resulted in a long development cycle and by the time it was ready for service use in 1956 the usefulness of antiaircraft shellfire was limited. In addition, the mounting proved to be so unreliable and hard to maintain that it was quickly withdrawn from duty and may have had the shortest service life of any weapon system ever used by the US Navy. It has been said that the only advantage of this mounting was that the gun shield let you work on them out of the weather. Most mounts were replaced within a few years with other weapons and the only US warship to retain the 3"/70 (7.62 cm) to the end of her career was USS Norfolk (DL-1).
As per the nomenclature change in the US Navy after World War II, the Mark 37 designation was for the mounting, not the gun itself. The gun went through four major revisions. The Mark 23 was a prototype used to test the operation of fully-automatic firing. The Mark 24 was similar but slightly lighter. The Mark 25 used a rapid-fire horizontal-wedge breech. The Mark 26 was the service version and was a water cooled monobloc type with a horizontally sliding breech mechanism. The barrel on this last Mark had a three caliber smoothbore section near the muzzle (Probertised) to reduce loss of velocity and to act as a flash suppresser. The breechblock opened mechanically as the gun recoiled and closed automatically as soon as the next round was loaded. The barrel is attached to the breech ring by interrupted threads (bayonet joint) and can be removed without dismounting the gun.
Two mountings from USS Norfolk (DL-1) still survive and are on display at the Boca Raton Community High School in Florida, USA.
|Designation||Weapon: 3"/70 (7.62 cm) Marks 23, 24, 25 and 26
Mounting: Mark 37 Mod 0
|Ship Class Used On||
Prototype was tested on Carpenter (DDE-825)
Northampton (CLC-1), Norfolk (DL-1) and Mitscher (DL-2) classes
|Date Of Design||1945 (initial) 1
|Date In Service||1958|
(not including breech block)
|Mark 23: 1,875 lbs. (850 kg)
Mark 24: 1,853 lbs. (841 kg)
Mark 25: 2,073 lbs. (940 kg)
Mark 26: 2,650 lbs. (1,202 kg)
|Gun Length oa||N/A|
|Bore Length||210 in (5.334 m) 2|
|Twist||Uniform RH 1 in 25|
|Rate Of Fire||90 - 100 rounds per minute|
|Weight of Complete Round||36 lbs. (16.4 kg)1|
|Projectile Types and Weights 2||AA Mark 34 Mod 1 3 : 15 lbs. (6.8 kg)|
|Propellant Charge||11.2 lbs. (5.1 kg) SPD
Cartridge weight (empty): 9.8 lbs. (4.4 kg)
|Cartridge||Mark 10 - 76.2 mm x 669R|
|Muzzle Velocity||3,400 fps (1,036 mps)|
|Working Pressure||22.5 tons/in2 (3,547 kg/cm2)|
|Approximate Barrel Life||2,050 rounds|
|Ammunition stowage per gun 4||Norfolk: 1,110 rounds
Mitscher: 1,000 rounds
Although munitions were a joint development, the USN and British cartridges were slightly different.
- ^"US Naval Weapons" incorrectly shows the complete round weight as being about 62 lbs. (28 kg).
- ^These guns fired a new, heavier projectile than did older USN 3" (7.62 cm) guns.
- ^ The fuze used for the AA projectile was normally a VT (proximity) type. It could be used in a contact mode by "paralyzing" the VT mechanism via a wiping action.
- ^There were a maximum of 77 ready rounds for each gun in the ammunition feed system.
|Weight||125,500 lbs. (56,926 kg)|
|Elevation||-15 / +90 degrees|
|Elevation Rate||35 degrees per second|
|Train||360 degrees 3|
|Train Rate||50 degrees per second|
|Gun recoil||15.62 in (40 cm)|
The requirement for a very high rate of fire for these guns meant a complicated loading system. Rounds were stowed horizontally in a 60-round cross feed, which loaded them into a 10-round elevator, which in turn loaded them into a 2-round transfer mechanism. The transfer mechanism fed the rounds into an ammunition hoist holding four rounds in clamps. This mechanism delivered the rounds up in front of the gun trunnion, over it, and into the rammer behind the gun. This very complex arrangement was needed in order to allow the gun to elevate to 90 degrees. The rammer itself was a power-operated cylindrical case which pivoted sideways to place the round just behind the breech, quickly moving forward and then abruptly stopping, catapulting the round into the breech.
This mounting was a base-ring type and the guns shared a single slide. The sliding-wedge breech blocks were arranged horizontally to permit very close placement of the guns, whose centers are only 32 inches (81.3 cm) apart, and thus minimize the dimensions of the mounting. Nonetheless, the Mark 37 mounting considerably outweighed a twin 5"/38 (12.7 cm) destroyer mounting and required weight compensation when installed on the Mitscher class destroyers.
- ^The single mounting did not enter service use. Unless otherwise noted, all information in this section is in regards to the Mark 37 twin mounting.
- ^USS Carpenter was originally a Gearing class (DD-710) destroyer that was first commissioned as an ASW destroyer (DDK) in 1949. As completed, she carried two twin 3"/50 (7.62 cm) mountings. In the early 1950s she was rebuilt as a test bed for the 3"/70 (7.62 cm) and she then carried two of these prototype weapons, mounted bow and stern. The 3"/70 (7.62 cm) mountings were removed in the mid-1960s and she was then fitted with a standard 5" (12.7 cm) Mark 38 twin mounting on the bow and a helicopter landing area in place of the stern mounting.
- ^The mounting uses slip rings and can be continuously rotated throughout 360 degrees.
- "Naval Weapons of World War Two" by John Campbell
- "US Destroyers: An Illustrated Design History" and "US Naval Weapons" both by Norman Friedman
- "Drill for 3 Inch Twin Mark 6 A.A. Mounting" produced by Weapons Division in Fleet School C.F.B. Halifax (1970)
- "Test Firings of 3 IN./70 Caliber Single Mount Mark 35 Mod 0, Report AD0499511" by E.S. Romero, Naval Proving Ground Dahlgren, Virginia
Special help from Leo Fischer and Anthony G. Williams