Italy
20 mm/65 Models 1935, 1939 and 1940
Updated 23 September 2012
This gun was air cooled and gas operated and seems to have been fairly efficient.  It was derived from the 13.2 mm (0.52") M1931.  Considered to have been a good weapon, although the low magazine capacity of 12 rounds meant frequent stoppages for change outs.  An unusual feature was that the spent round was reinserted into the magazine.

Another 20 mm weapon used by the Italians during World War II was the 20 mm/70 built by Scotti-Isotta Fraschini.  This was used on the twin RM1939 mounting and a disappearing submarine mounting.  This weapon had similar performance to the Breda.
WNIT_20mm-65_Breda_pic.jpg

20 mm/65 Breda guns in Twin RM1935 mounting on a MTB
Photograph copyrighted by Elio Andò
WNIT_20mm-65_Breda_single_pic.jpg

20 mm/65 from a Navigatori class destroyer
Image courtesy of Museo Tecnico Navale della Spezia
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Gun Characteristics
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Designation 20 mm/65 Models 1935, 1939 and 1940
Ship Class Used On Almost all
Date Of Design 1935
Date In Service 1937
Gun Weight 159 lbs. (72 kg)
Gun Length oa N/A
Bore Length about 51.2 in (1.300 m)
Rifling Length N/A
Grooves N/A
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire 240 rounds per minute cyclic
about 120 rounds per minute practical
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Ammunition
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Type Fixed
Weight of Complete Round 0.705 lbs. (0.32 kg)
Projectile Types and Weights HE - 0.295 lbs. (0.134 kg)
Bursting Charge N/A
Projectile Length N/A
Propellant Charge 1.34 oz. (0.038 kg)
Muzzle Velocity about 2,756 fps (840 mps)
Working Pressure 17.8 tons / in2 (2,800 kg / cm2)
Approximate Barrel Life 5,000 rounds
Ammunition stowage per gun about 1,500 rounds
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Range
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Elevation With 0.295 lbs. (0.134 kg) Round
Range @ 45 degrees 6,000 yards (5,500 m) max.
2,730 yards (2,500 m) effective
AA Ceiling 9,500 feet (2,900 m)
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Mount / Turret Data
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Designation Model 1935:  Stabilized Twin
Model 1939:  Single Mounting
Model 1940:  Single Mounting
Weight  Model 1935:  5,137 lbs. (2,330 kg)
Model 1940:  689 lbs. (312.5 kg)
Elevation Singles:  -10 / +90 degrees
Twins:  -10 / +100 degrees
Elevation Rate Manual operation, only
Train about +120 / -120 degrees
Train Rate Manual operation, only
Gun recoil N/A
Notes: 

1) Model 1935 was a twin mount with a stabilized line of sight.  The left gun was mounted diagonally above the right gun (see picture above).

2) Model 1939 was a little-used single mounting.

3) Model 1940 was a free-swinging single mounting widely used on smaller ships.

4) Gun crew for twin mounts was one aimer and four loaders.
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Data from
"The Italian Navigatori Class, 1926" article by Elio Andò in "Warship Special 2:  Super Destroyers" edited by Antony Preston
"Naval Weapons of World War Two" by John Campbell
"Battleships:  Axis and Neutral Battleships in World War II" by W.H. Garzke, Jr. and R.O. Dulin, Jr.
"Anatomy of the Ship:  The Cruiser Bartolomeo Colleoni" by Franco Gay and Valerio Gay
Page History

21 September 2008 - Benchmark
26 May 2012 - Updated to latest template
23 September 2012 - Added picture of museum gun