Bofors 40 mm/70 Model 1948, Model 1958, Sea Trinity, Mark 3 and Mark 4
Updated 31 October 2011

Developed from the Bofors 40 mm/60 Model 1936, the 40 mm/70 was originally designed to replace the older weapon's twin mountings with single mounts by doubling the rate of fire (ROF).  Newer, faster-firing versions of this weapon are still in production by Breda as well as Bofors.  All of these 40 mm/70 guns use gravity feed magazines whose designs are directly descended from those for the 40 mm/60 Model 1936.

Although an impressive AA gun in the late 1940 to 1960 time frame, the value of such MG AA weapons has decreased since World War II with the increasing sophistication of stand-off anti-ship weapons such as the USA's Harpoon.  For that reason, Bofors has developed more sophisticated ammunition such as the 3P round which can be set for point contact or proximity detonation.  These improvements, together with a better FCS, allow these weapons to be used against sub-sonic cruise missiles although they are more often intended to be used against small surface craft.

In the late 1980s Bofors upgraded the design of the 40 mm/70 to allow higher rates of fire and improve reliability.

 The Mark 3 can be integrated with any analog or digital fire control system.  This mounting is normally unmanned and controlled from the combat information center but can be locally controlled from an on-mount operator's console for aiming and will remain fully stabilized by the gun's local gyros.  In case of total power supply failure, the gun can be layed and fired manually.  A radar on the gun barrel is used for correcting the FCS solution.

The Mark 4 is an improved version of the Mark 3 and has replaced the hydraulic drives of earlier designs with all electric motors which reduces the flammability of the mounting and improves reliability.  Neither of these mountings have any deck penetration except for electrical connections.

Breda has produced their own versions of the 40 mm/70 under license since 1969.  The company has specialized in automatic loading mechanisms which are also used for their own designs, as can be seen on the 40 mm/70 OTOBreda page.

Bofors was purchased by United Defense which was in turn bought by BAE Systems.


40 mm/70 Sea Trinity Mockup
Picture copyrighted by Bofors Defence

Click here for additional pictures
Gun Characteristics
Designation 40 mm/70 (1.57") Model 1948, Model 1958, Sea Trinity, Mark 3 and Mark 4
Ship Class Used On Model 1948:  Tre Kronor and other Swedish Warships

Model 1958:  German warships since the 1960s

Sea Trinity:  N/A

Mark 3:  N/A

Mark 4:  N/A

Date Of Design 1948
Date In Service Model 1948:  1952
Others:  N/A
Gun Weight about 386 lbs. (175 kg)
Gun Length N/A
Bore Length about 110.2 in (2.800 m)
Rifling Length N/A
Grooves 16
Lands N/A
Twist N/A
Chamber Volume N/A
Rate Of Fire Model 1948 and Model 1958:  240 rounds per minute. 

Sea Trinity:  300 rounds per minute

Mark 3:  330 rounds per minute

Mark 4:  300 rounds per minute

Type Fixed
Weight of Complete Round PFHE - 5.4 lbs. (2.5 kg)
HE-T - 5.3 lbs. (2.4 kg)
40 3P - 5.5 lbs. (2.5 kg)
Length of Complete Round 21.0 in (53.34 cm)
Projectile Types and Weights PFHE - 1.94 lbs. (0.88 kg)
HE-T - 2.12 lbs. (0.96 kg)
40 3P - 2.15 lbs. (0.975 kg)
Bursting Charge PFHE - N/A
HE-T - 0.254 lbs. (0.115 kg)
40 3P - >3,000 fragments propelled by PBX
Projectile Length 8.3 in (21 cm)
Complete Round Length 21.4 in (53.44 cm)
Propellant Charge 1.07 lbs. (0.485 kg)
Cartridge 40 mm x 365R
Muzzle Velocity HE-T - 3,297 fps (1,005 mps)
PFHE - 3,363 fps (1,025 mps) 
40 3P from L/70 - 3,320 fps (1,012 mps)
40 3P from Trinity - 3,610 fps (1,100 mps)
Working Pressure 21.8 tons/in2 (3,250 kg/cm2)
Approximate Barrel Life Mark 3 and Mark 4 - up to 5,000 rounds
Ammunition stowage per gun
(see Note 1)
Model 1948 - N/A
Sea Trinity - 99 rounds per magazine
Mark 3 - 101 rounds per magazine
Mark 4 - 30 rounds on gun plus 70 rounds in a ready-use magazine

1) The Mark 3 uses a dual compartment 101-round magazine.  The magazine can be rapidly switched to provide options between the latest Bofors 6 mode programmable 3P rounds and other 40 mm L/70 ammunition types.  The Mark 4 has a 30-round magazine on the gun which is automatically replenished by a 70-round intermediate magazine.

2) Bofors 40 mm 3P ammunition fuze options allow 3 proximity modes as well as settings for time, impact and armor piercing requirements. 

3) Proximity fuzing is greater than 33 feet (10 m) for aircraft, 16 feet (5 m) for missiles and 10 feet (3 m) for sea skimmer targets.

4) Tracer burns for about 4 seconds.

Model 1948 Range
Elevation With 1.94 lbs. (0.88 kg) PFHE
Maximum Horizontal Range 13,120 yards (12,000 m)
AA Ceiling 13,120 feet (4,000 m)
Sea Trinity Range
Effective AA range 3,280 yards (3,000 m) against low flying aircraft
Effective CIWS range 2,740 yards (2,500 m) against a sea skimming missile
Mark 3 and Mark 4 Range
Maximum 13,670 yards (12,500 m)
Effective range against surface targets 9,140 yards (10,000 m)
Effective CIWS range 2,740 yards (2,500 m) against a sea skimming missile
Mount / Turret Data
(see Notes)
Model 1948
Sea Trinity
Mark 3
Mark 4
Weight Model 1948:  Weight between 6,173 to 7,275 lbs. (2,800 to 3,300 kg) depending upon the version.

Sea Trinity:  8,157 lbs. (3,700 kg)

Mark 3
   Above deck (excluding ammunition):  7,700 lbs. (3,500 kg)
   Below deck:  770 lbs. (350 kg)

Mark 4 (excluding ammunition):  5,500 lbs. (2,500 kg)

Elevation Model 1948:  -5 / +90 degrees

Sea Trinity:  -20 / +80 degrees

Mark 3 and Mark 4:  -20 / +80 degrees

Rate of elevation Model 1948:  N/A

Sea Trinity:  45 degrees per second

Mark 3:  57 degrees per second

Mark 4:  N/A

Train 360 degrees continuous (slip rings)
Rate of Train Model 1948:  N/A

Sea Trinity:  85 degrees per second

Mark 3:  92 degrees per second

Mark 4:  N/A

Gun Recoil Model 1948 and Model 1958:  9.8 in (25 cm)
Others:  N/A

1) Bofors 40 mm Sea Trinity was conceived as a lightweight CIWS with low magnetic signature and enclosed in a manned or unmanned stealthy gunshield.  Trinity Spartan uses an off-mount fire control while Trinity normally has an on-board FCS with several options including the Goalkeeper FCS or the Ericsson Eagle radar.

2) The Model 1948 and Sea Trinity mountings employ open shields with the option of a GRP weather cover.  SAK-40/L70-315 is manually (hand) operated.  SAK 40/L70-300 may use the Breda 144 round magazine.  SAK 40/L70-350 is power operated, fully stabilized and remotely controlled.  SAK 40/L70-600 employs the Sea Trinity 99 round magazine and has a 330 RPM rate of fire.  All German Navy guns have been upgraded to this standard.  British Royal Navy plans for single, twin and sextuple versions (Mark's 10, 11 and 12) of the 40 mm/L70 gun were cancelled in 1957 in favor of Sea Cat.  The Mark 3 is a Stealthy mounting based on the Sea Trinity.  Mark 4 includes a video camera on the gun barrel as well as a radar antenna.

3) The Mark 3 operates from 440 Vac 60 Hz three phase power.

4) The Model 1958 was a modernized Model 1948 first used by the Bundesmarine (Federal Republic of Germany) on former Fletcher class (DD-445) destroyers supplied under the US military assistance program.  Used in both single and twin versions and still in use today on Schnellboots and Minecraft.

Data from
"The Naval Institute Guide to World Naval Weapon Systems 1991/92" by Norman Friedman
"Jane's Ammunition Handbook:  Ninth Edition 2000-2001" edited by Terry J. Gander and Charles Q. Cutshaw
"Rapid Fire" by Anthony G. Williams
"Bofors 40 mm Mk4 Naval Gun System Datasheet" produced by BAE Systems
BAE Systems video presentation on the Mark 4
BAE Systems Press Releases
Bofors Defence Press Releases
United Defense Press Releases
Special help from Matthew Rodchenko, Joakim Wohlfeil, Mark Fitzpatrick, Leo Fischer and Hans Migielski of Fregatten Köln
Page History

20 December 2007 - Benchmark
31 October 2011 - Added information about Mark 4