United States of America
1.1"/75 (28 mm) Mark 1 and Mark 2
Updated 03 April 2012

This weapon was developed when an October 1928 Board convened by BuOrd decided that the 0.50" (12.7 mm) machine gun was too light a weapon for future air defense.  Design began in March 1929 and initial testing with a single barrel weapon was conducted between March and May 1931.  BuOrd decided to use these weapons in a quadruple mount as the combined rate of fire approached that of a single 0.50" (12.7 mm) MG of roughly 500 rounds per minute.  It was felt that this rate of fire would result in a disabling amount of hits on even a fast-diving plane.  The projectiles for the 1.1" (28 mm) differed substantially from those for the 0.50" (12.7 mm) MG by having an explosive filler which was set off by a "super-quick" fuze.  A 1934 report to the Navy General Board concluded that a single hit on any part of an aircraft would probably result in a forced landing.

This weapon was rushed into production as soon as it had passed its proving ground tests and was widely used on US warships before and during the first years of World War II.  During early service use, it was found to be unreliable, prone to jamming and ineffective as an AA weapon.  Never popular, they were rapidly replaced by the 20 mm Oerlikon and 40 mm Bofors AA weapons during World War II and by January 1945 they were in active service only on a few smaller ships.  All surviving guns and mountings were ordered scrapped in 1945 but some of the power drives were retained as being suitable for the twin Bofors mountings.  In retrospect, it would appear that these defects were little more than teething problems and that the basic design was sound.  However, this weapon still lacked the range and larger bursting charge of the 40 mm Bofors.

Construction was a monobloc barrel with chromium plating both internally and externally.  Mod 1 had a threaded muzzle to accept a flash suppresser.  Mark 2 was an experimental gas operated version that did not enter service.

The quad mountings were used with the Mark 44 Director, which was the USN's first off-mount director for automatic anti-aircraft weapons.  This was a simple "dummy gun" type director and was manually trained and elevated.  The Mark 44 contained optics and a spotting glass, but did not have any lead angle computing elements such as were found in later directors for automatic weapons such as the Mark 51.  The Mark 44 simply removed the operator away from the noise and smoke of the weapon, providing a clearer line of sight to the target.  However, the Mark 44 did provide a basis for further development for the remote control of automatic weapons and greatly influenced the design of the Mark 51 and later small AA directors.


1.1" (28 mm) Quad Mount
An interesting view that shows the gearing that allowed the guns to be slewed from side to side.  This allowed this mounting to track a dive bomber even at very high gun elevations, where the normal train axis would have little effect.  The hoses connect the water recirculating pump to the barrel cooling jackets.

Click here for additional pictures
Gun Characteristics
Designation 1.1"/75 (28 mm) Mark 1
Ship Class Used On Destroyers and larger ships 1930s - 1940s
Date Of Design 1929
Date In Service
(see Note)
Gun Weight 556 lbs. (252 kg) (without breech)
Gun Length oa 119.6 in (3.037 m)
Bore Length 82.0 in (2.083 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume 10.5 in3 (0.172 dm3)
Rate Of Fire Cyclic:  150 rounds per minute
Practical:  About 100 rounds per minute
Note:  A prototype was produced in 1931 and limited production started in 1934 with the first unit being completed in April 1935.  Although service introduction was in 1936, this weapon was not available in quantity until 1940.
Type Fixed
Weight of Complete Round 1.9 lbs. (0.86 kg)
Projectile Types and Weights
(see Note 1)
HE-T Mark 1 Mods 0 to 28 - 0.917 lbs. (0.416 kg)
HE-T/SD Mark 1 - 0.917 lbs. (0.416 kg)
HE-T/SD Mark 2 Mods 0 and 1 - 0.917 lbs. (0.416 kg)
Bursting Charge HE-T - 0.037 lbs. (0.017 kg) Explosive D
HE-T/SD - 0.034 lbs. (0.015 kg) Explosive D
Projectile Length HE-T Mark 1 - 5.8 in (14.7 cm)
HE-T/SD Mark 1 - 5.8 in (14.7 cm)
HE-T/SD Mark 2 - 5.7 in (14.5 cm)
Cartridge Case Type, Size and Empty Weight Mark 1 - Brass, 28 x 199 mm, 0.688 lbs. (0.31 kg)
Propellant Charge 0.265 lbs. (0.120 kg)
Cartridge - 0.883 lbs. (0.444 kg)
Muzzle Velocity New Gun:  2,700 fps (823 mps) 

Average:  2,600 fps (792 mps)

Working Pressure 16.0 tons/in2 (2,520 kg/cm2)
Approximate Barrel Life N/A
Ammunition stowage per gun N/A

1) The many Mods of USN ammunition were primarily bookkeeping designations used to indicate the manufacturer.  The HE-T/SD Mark 1 rounds were HE-T Mark 1 projectile bodies modified by drilling a hole through the wall separating the tracer and the burster.  All Mark 2 rounds used a self-destroying tracer as originally designed as shown in the picture above.  When the tracer detonated, it set off the burster of the projectile, destroying the projectile.  The actual USN designations were as follows:

   HE-T Mark 1:  1.1-Inch AA Mk 1
   HE-T/SD Mark 1:  1.1-Inch AA Mk S.D. 1
   HE-T/SD Mark 2 Mod 0:  1.1-Inch AA Mk 2 Mod 0
   HE-T/SD Mark 2 Mod 1:  1.1-Inch AA Mk 2 Mod 1

2) Ammunition was fed in eight-round clips, two to each gun.  The gun fed from one clip at a time, allowing for empty clip changeouts while the gun kept firing from the other clip.  A loaded eight-round clip weighed about 34 lbs. (15.4 kg).

3) Tracer burnt for about 3,000 yards (2,750 m).  The projectiles were fitted with super-sensitive contact fuzes with a short delay.  It was claimed in a November 1934 report to the General Board that this fuze was so sensitive that it would "function on impact with a single thickness of airplane wing fabric and which has such a short delay that the projectile will explode in the wing section.  The damaging effect of this projectile is remarkable.  A [single] hit on any part of the plane will probably cause a forced landing."

4) Bourrelet diameter was 1.085 inches (2.76 cm).

USN 1.1 inch Ammunition Color Codes
These projectiles were originally issued unpainted except for two dots below the fuze, which were as follows:

   - Yellow Dot indicating that the burster was Explosive D
   - Red Dot indicating that a Tracer was used

A color marking system was later proposed which may have used on ammunition issued late in the war.  This marking system is shown in the following table.

Type Color
HE-T Light Gray with White Band
HE-T/SD Dark Green with White Band
BL&P Red all over
BL&T Red all over with White Band
Elevation 0.917 lbs. (0.416 kg) HE Mark 1 Shells
Range @ 10 degrees 5,300 yards (4,846 m)
Range @ 15 degrees 6,100 yards (5,578 m)
Range @ 20 degrees 6,600 yards (6,035 m)
Range @ 25 degrees 6,900 yards (6,309 m)
Range @ 40.9 degrees 7,400 yards (6,767 m)
AA Ceiling 19,000 feet (5,791 m)
Mount / Turret Data
Designation Quadruple Mount
   Mark 2 Mod 2
Weight  Mark 2 Mod 2:  10,500 lbs. (4,763 kg)

Other mountings weighed between 4.7 and 6.25 tons (4.78 - 6.35 mt)

Elevation -15 / +110 degrees
Elevation Rate 24 degrees / second
Train 360 degrees
Train Rate 30 degrees / second
Gun recoil 3.25 in (82.55 mm)
Side-to-side slewing - 30 / +30 degrees

1) It was considered that the early quadruple mounts suffered from too much vibration, so they were modified by adding large steel stiffening plates between the carriage and the base of the mounting.  Later mounts incorporated this change and lacked the side-to-side slewing feature.

2) RPC was fitted to all Mark 2 mountings.

3) In 1941 - 42, the normal allocation was four mountings for larger ships and one or two for destroyers.  However, USS Lexington (CV-2) carried a total of 12 mounts when lost in 1942.

Data from
"Naval Weapons of World War Two" by John Campbell
"US Naval Weapons" by Norman Friedman
"Battleships:  United States Battleships, 1935-1992" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"Rapid Fire" by Anthony G. Williams
"US Navy Projectiles and Fuzes" published by U.S.N.B.D.
"Ammunition:  Instructions for the Naval Service:  Ordnance Pamphlet 4 - May 1943" by Department of the Navy
"U.S. Explosive Ordnance:  Ordnance Pamphlet 1664 - May 1947" by Department of the Navy
Gene Slover's Navy Pages
Special help from Anthony G. Williams
Page History

16 May 2008 - Benchmark
14 January 2011 - Added data reference
03 April 2012 - Added pictures page