Northwest Thailand during World War II

N 18°32.300
E 99°03.000
Ban San Khayom Bridge (Th: สะพานบานสันกะยบม / Jp: バンサンカヨム橋)
Page 8 of 11

Railroad Sta


Text Notes

Supplemental information: the nominal 60mm hole

60mm hole

The greatest problem with the hole at Point G is its diameter. The 60mm± doesn't readily fit any caliber gun present in Thailand during WWII. A wide range of weaponry was reviewed because it was suggested that an HE (High Explosive) round from a smaller caliber gun might have exploded on impact and possibly enlarged Hole G substantially beyond the projectile's diameter. That review is presented below, but in retrospect, the petaling at the hole does not suggest an explosion enlarged the hole. In addition, some HE rounds had fuze delays which would have prevented the shell from detonating at impact. Problems with a hypothesis involving an HE round are discussed below.


Calibers, 20mm - 60mm, US (Allied) military arsenals:

Aircraft mounted:

20mm (3/4 in):

High explosive (HE) ammunition for an HS.404 autocannon, used in Lockheed P-38 Lightnings and Bristol Beaufighters.

37mm (1‑7/16 in):

The 37-mm Automatic Gun M4 was mounted in aircraft.[L13]
Ammunition used was the M54 [HE] projectile[L14]
The projectile consists of . . . Point-detonating Fuze M56 [L15]

The Airacobra [P-39] was designed around the 37mm M4 autocannon.[L16] However, the P-39 is not recorded as having been used in Southeast Asia.

Vickers Class "S" 40mm (1.57 in)

. . . The gun was originally intended as a bomber defensive weapon . . . but when the need to attack tanks from the air was identified, the "S" gun was chosen and special armour-piercing ammunition developed. . . . Two underwing guns . . . were fitted to Hawker Hurricane IID fighters.

. . . In 1944, the aircraft served in the Far East, mainly firing HE ammunition against road and river transports.[L16a]

. . . the type . . . saw success in southeast Asia using high explosive shells to attack Japanese transportation routes. . . .[L16b]

. . . [28 April 1944] The newly arrived detachment of 20 Squadron Hurricane IIds commenced operations on this date, but having only armour-piercing 40mm cannon ammunition, they found it difficult to confirm the results of attacks on rivercraft with these projectiles.[L16b1]

. . . [June 1944] [20 Squadron] "claimed the destruction of 12 tanks, 501 sampans, 336 dirsties, 348 dugout canoes . . . . by the end of the June."[L16b2]

Mosquito Mk XVIII 57mm

The type was built in several variants, including the FB.XVIII fighter bomber. Also known as the "Tse Tse Fly," this particular model was armed with four 0.303-inch caliber machine guns and a single 57-mm cannon in the nose. The 57-mm cannon was based on a standard Army light artillery piece called the 6-pounder but modified with a Molins automatic loader. The entire system weighed about 1,800 lbs (815 kg). The aircraft carried 25 rounds of ammunition, weighing 7 lbs (3.2 kg) each, and the automatic loader could fire the entire magazine in just 20 seconds.

The aircraft was originally designed as an anti-tank weapon, but was instead used for anti-shipping and anti-submarine duties as well as to attack shore installations. However, rockets were found to be better suited to the task, so the cannon was later supplemented by an armament of two 500-lb bombs or eight 60-lb rockets. Only 27 examples were built, the first seeing action in October of 1943.[L16c]


40mm (1‑9/16 in)

. . . sometimes called the Bofors gun . . .[L17]
[land based] . . . [anti-aircraft] very effective against dive bombers and low-flying aerial targets. . . also effectively used against armored ground targets.[L18]
Fuzes seem to be all of the impact type "direct action".[L19]

In any case, such Allied land weaponry would not have been present in Thailand prior to the end of the war.

57mm (2‑1/4 in)

The 57-mm Gun M1 . . . [land based] has been successfully employed as an antitank weapon. . . There are only two types of ammunition provided: the armor-piercing and the practice.[L20]

Again, such Allied land weaponry would not have been present in Thailand prior to the end of the war.

Calibers, 20mm - 60mm, IJA and RTA military arsenals:

Aircraft mounted:

Ho-401 cannon[L21]

Ho-401 was a Japanese aircraft autocannon that saw limited, if any, use during World War II. It was a large-caliber version of the 37 mm Ho-203 cannon.

Caliber: 57 mm (2.25 in)
Ammunition: 57 x 121R (1,500 g)

Following up on the Wikipedia's mention of the 37 mm Ho-203 cannon:[L22]

Ho-203 was a Japanese autocannon that saw considerable use during World War II. . . It was operationally used only as the nose gun of the Ki-45 KAI heavy fighter, the anti-bomber workhorse of the Imperial Japanese Army, and tried out in the upper fuselage of the III-KAI variant of the Mitsubishi Ki-46 Dinah.

Caliber: 37 mm (1.45 in)
Ammunition: 37 x 112R (475 g)

There is no record that the Ki-45 was used in Thailand.

On the assumption that the 57mm Ho-401 autocannon might have followed the Ho-203 onto a Ki-46 used in Thailand:

Per an IJAAF Order of Battle for the Burma Theatre of Operations, dated 20 March 1942, the 51st Independent Chutai, a recon unit, not a combat-proper unit, assigned to Lampang, had a complement of 5 Ki-46s.[L22a]

However, a search found no Ki-46 "variant" listed with an Ho‑401 or with a 57mm weapon.[L22b]

The Ho-401 cannon was installed in the Ki-102 "Randy".[L22c] That aircraft never operated south of Okinawa.[L22d]

The Ho-401 cannon was installed in an experimental Rikugun Ki‑93, which was destroyed during the aircraft's development.[L22e]

Ho-402 cannon

Wikipedia provides only this listing.[L23]

An interceptor version of the experimental Ki-93 (Ki-93-1a) carried a 57mm Ho-402; but there is no information about the aircraft's operational history, if any.[L23a]


20mm caliber weapons are not included for reasons cited above. It is important to note that a resourceful Thai, charged with defending the Kaeng Luang Bridge from air attack, and lacking an anti-aircraft weapon, used an anti-tank gun to down a B-25 during its approach for a bombing run on the bridge on 21 November 1944. Unfortunately its caliber was not recorded.

Model 96 25mm anti-aircraft / anti-tank automatic cannon

The Type 96 . . . has been found in dual and triple fixed mounts, emplaced customarily around air strips for antiaircraft defense. . . . was . . . used in land bases in the Japanese Empire and in the Japanese overseas combat fronts. These weapons were also used as anti-tank guns in some defensive actions in Pacific theaters and against land objectives in southeast Asia/Chinese mainland during the Pacific War. . . . The weapon is furnished with high explosive tracer, high explosive, and armor piercing tracer ammunition.[L23b]

Model 94 (1934) 37mm gun

. . . referred to by the Japanese as an "Infantry rapid fire gun" . . . an infantry close support weapon firing both high explosive and armor piercing high explosive ammunition.[L23c]

40mm single & dual anti-aircraft and anti-tank automatic cannon

. . . a Vickers type recoil-operated . . . automatic or semi-automatic machine cannon. . . . Ammunition: Armor piercing high explosive, high explosive with time fuze, high explosive with point detonating fuze, and tracer.[L23d]

Model 1 (1941) 47mm gun

. . . an antitank weapon of modern design. . . . Ammunition: Armor piercing high explosive and standard high explosive shells . . . .[L23e]

Type 98 50mm mortar

The Type 98 50 mm mortar fired a formidable stick bomb weighing nearly 10 pounds and containing an explosive charge of approximately 7 pounds of picric acid in rectangular blocks. The body of the bomb, made of sheet metal (three-sixteenths of an inch thick), is rectangular in shape 4 1/2 by 4 1/2 by 6 1/2 inches) and is painted black. A hardwood stick 21 inches long and 1.91 inches in diameter fits into a socket in the base of the body, and is held in place by nails or screws.[L24]

Conclusion: The projectile designed for this mortar could not produce the clean circular hole found at Point G.

Type 2 57mm anti-tank gun[L25]

Prototype tested in 1941–1943; project was cancelled because of the appearance of Allied heavy tanks.[L25a]

57mm tank guns

Light tank: Type 4 Ke-Nu. Not in production until 1944, the tank saw only "limited" action In Manchukuo during the Soviet invasion in 1945.[L25b]

Most medium tanks[L25c] had a turret-mounted 57mm gun, including these types, the last of which, Type 97, was the tank most widely used by the IJA in WWII:

89A (1929)
89B (1929)
94   (1934)
97   (1937)[L25f]

The only gun discussed in detail for these models in the US's Handbook on Japanese Military Forces (1944) was that used in the Type 97 tank, its 57 mm gun also being numbered Type 97.[L25g] "Tube length" (barrel?) was 0.955m (3 foot 1.6 inches), with a chamber length of 0.130 (5.1 inches). Maximum elevation was 45°, while traverse was limited to 10° left and right (while the turret itself had 360° movement). Ammunition was HE and APHE. The gun is generally described as:

. . . a short barrelled, medium velocity weapon that would be more suitable for employment against ground troops.

However, Akia Takizawa offers this description of IJA 57mm guns:[L25h]

Type 90 57mm Tank Gun
    Caliber : 57 mm
    Barrel Length : 1.050 m (L18.4)
    EL Angle of Fire : -15° to +20°
    AZ Angle of Fire : 20°
    Muzzle Velocity : 350 m/sec
    Penetration : 20 mm/500 m
    Used by Type 89 Medium Tank

Type 97 57mm Tank Gun
    Caliber: 57 mm
    Barrel Length: 1.057 m (L18.5)
    EL Angle of Fire: -15° to +20°
    AZ Angle of Fire: 20°
    Muzzle Velocity: 350 m/sec
    Penetration: 20 mm/500 m
    Used by Type 97 Medium Tank

Wikipedia's article provides additional details about the Type 97 57mm gun:[L25i]

    Calibre: 57 mm
    Barrel Length: 1.05m (18.4 calibers)
    Muzzle velocity: 355.3 m/s
    Elevation (as mounted on Chi-Ha): -15° to 2°
    Penetration: 25 mm/1,000 m
    Shell: AP and HEAT

No IJA medium-sized tanks are recorded as having been stationed in Thailand.

Allied weaponry

There is apparently almost no possibility that the IJA may have captured Allied 57mm weaponry. The British so-called "Ordnance QF 6 pounder" did not come into use until after the IJA invasions of Malaya, Singapore, and Burma; and there was little contact with the British again until Imphal and Kohima, after which defeated IJA were unlikely to have carted back any gear captured (if British forces had even used them there, and that's uncertain). The US started issuing its own manufactured 57mm Gun M1 in 1943, but it was only used on the Western Front. There is no record of it having been supplied to Nationalist Chinese forces.[L25j]


As noted, it has been hypothesized that an HE (High Explosive) round hitting the bridge member might have exploded at impact producing a hole larger than the caliber of the projectile itself.[L26] That it apparently did no further damage might be attributable to the expansive force of the explosion continuing into the relatively unconfined space of an open structural steel member.

On reflection, though, this scenario is unlikely. HS.404 HE shells normally had time-delay fuzes, with timing typically a millisecond or somewhat less.[L27] Using the HS.404 initial projectile velocity of 880 m/s, the distance covered in a millisecond would have been 880mm (2.6 feet). Admittedly, passing through the steel target would have slowed the shell; nonetheless the steel was not hardened and was comparatively thin, so the effect is assumed to have been minor. Thus, if an HS.404 HE round hit Point G, it would have detonated almost a meter beyond the steel target, and could not have enlarged the hole. Should the velocity have been halved by the time the projectile hit the bridge, it would still have detonated well after passing through the target steel.

Further, had any HE shell hit and detonated on impact at Point G and enlarged the hole, the petaling should have reacted to the explosion by tending to peel back onto the plate. One photo of Point G fairly clearly shows that all the petals were roughly parallel, and in that particular view the petals appeared to be uniformly perpendicular to the plate.

As an alternative solution, it was suggested changing the chronological "playing field"; ie, expanding the period in which the damage occurred to postwar so as to expand the possible field of weapons producing the holes, particularly for Hole G.[L28] While Thailand's recent history records numerous military coups, plus border conflicts with Burma and Cambodia, and a major siege of a Communist stronghold farther south, there is no record of postwar damage to "brick and mortar" infrastructure in this area.

One infamous domestic confrontation in Thailand never came to blows: a tiff between an army and a police general which was publicized originally in 1956:[L29]

The Thai press called it the "Opium War" and its protagonists the "yellow uniforms" and the "green uniforms". The engagements were subterranean for the most part, but occasionally there was a clash that could not be kept secret. One of these occurred in 1950 at Lampang, the railroad center just south of [Chiang Rai] The police in their yellow uniforms halted an army convoy carrying several tons of opium from the border to a railroad rendezvous at Lampang. The army troops not only refused to surrender their cargo, but set up machine guns and defied the police to take it. The police brought out their heaviest guns and took shelter facing their rivals. No shots were fired, but for forty-eight hours Lampang nervously watched the preservers of internal order face with deadly enmity the defenders of the country's borders. The impasse ended bloodlessly with the arrival from Bangkok of Phao Sriyanond, then a lieutenant general and deputy director general of police, and Maj Gen Sarit Thanarat, then commander of the First Army in Bangkok, now a full general and commander-in-chief of the army. These friendly rivals took possession of the opium and escorted it to the capital, where it disappeared into an ocean of official silence.

The location of this standoff was somewhere north of Lampang on the Phahonyothin Road from Chiang Rai, which is far distant from the San Khayom Bridge.

An observation was made that non-HE projectiles can produce holes of larger diameter than the projectiles themselves.[L30] In this instance, the projectile next smaller to the ~60mm of Hole G in common use at that time was 40mm; that's a rather large jump, an increase of 50% in diameter (and an increase of 125% in circular area displaced), for a 40mm projectile to produce a 60mm hole.

Nonetheless, the example cited in the post, a 4mm projectile making a 6mm diameter hole in armor plate is in exactly the same proportion as at Point G with 40mm and 60mm, respectively. However, such an increase in size would be partly a function of the resistance of the target. Hole G represents a perforation in a thin plate of comparatively soft structural steel, not hardened armor plate: it could be surmised that the lesser resistance offered by these conditions would not support creation of a hole 50% larger than the projectile. But the suggestion does provide a solution if the 60mm (by one measure) was produced by a somewhat smaller projectile --- for example, one of 57mm.

Thus it seems that a 57mm projectile most likely made Hole G. However, as noted at the beginning of this page, there is no record of any weapon having been used by either side in northwest Thailand that could have made the hole. With that caveat, the short list of candidates below includes only those judged otherwise probable:

Allied Mosquito Mk XVIII
Allied Anti-tank Gun M1
IJA medium tanks

(Japanese equipment listed above as in development is not included.)

With the preamble above, this line of thought is continued in the discussion at Possible sources for the hole at Point G.


This section on supplemental information
continues on next page about a different topic














L01-L12. (deleted).






L13.^ Ammunition Inspection Guide (Washington: US War Department, 02 Mar 1944), p 331.

L14.^ ibid, p 355.

L15.^ ibid.

L16.^ Wikipedia Bell P-39 Airacobra.






L16a.^ Wikipedia: Vickers S.

L16b.^ Aerospaceweb: Airborne Guns: Hurricane Mk IID and IV: 40-mm.


L16b1.^ Shores, Christopher, Air War for Burma, Vol III, The Allied Air Forces Fight Back in South East Asia (London: Grub Street, 2005), p 214:

L16b2.^ Shores, ibid, p 239










L16c.^ Aerospaceweb: Airborne Guns: Mosquito Mk XVIII: 57-mm.




L17.^ Ammunition Inspection Guide, ibid, p 363.

L18.^ ibid.

L19.^ ibid, pp 369-378.





L20.^ ibid, p 380.







L21.^ Wikipedia: Ho-401 cannon.





L22.^ Wikipedia: Ho-203 cannon.









L22a.^ Shores, Christopher, et al, Bloody Shambles, Vol 2 (London: Grubb Street, 1993), Table 13, p 347. The unit's function was reconnaissance (p 352).

L22b.^ Wikipedia: Ki-46.

L22c.^ The Pacific War Online Encyclopedia: Japanese 57mm Ho‑401 Cannon.

L22d.^ The Pacific War Online Encyclopedia: Ki-102 "Randy".

L22e.^ RCG Forum: Rikugun Ki-93.

L23.^ Wikipedia: List of weapons of World War II Japanese aircraft.

L23a.^ RCG Forum: ibid.









L23b.^ Wikipedia: Type 96 25 mm AT/AA Gun.


L23c.^ Handbook on Japanese Military Forces (Washington: USGPO, 1944) [hereafter Handbook], pp 217-218.



L23d.^ ibid, p 218.




L23e.^ ibid, pp 230, 234.






L24.^ Wikipedia: Type 98 50mm Mortar



L25.^ Wikipedia: List of Japanese military equipment of World War II. and 試製五十七粍戦車砲.

L25a.^ Wikipedia:
  Anti-tank Guns and

L25b.^ Wikipedia: Type 4 Ke‑Nu.

L25c.^ Handbook, ibid, pp 246-249.

L25d & e (deleted).


L25f.^ Imperial Japanese Army Page: Type 97 Medium Tank "Chi-Ha"


L25g.^ Handbook, ibid, pp 255-256.





L25h.^ Imperial Japanese Army Page: Type 97-57mm Tank Gun.











L25i.^ Wikipedia: Type 97 57mm Tank Gun.









L25j.^ Wikipedia: Ordnance 6-pounder in US service (originally only tentatively suggested in j-aircraft forum by Greg Hill, post 17, 2146 03 Oct 2014




L26.^ Suggested by several, firstly per post 2 by LWD  on 1753 05 Mar in Axis History Forum topic "Aircraft guns and bullet behavior".


L27.^ War Thunder Online Forum: Aircraft Gunnery and Ballistics. Use of delay fuzes in HS.404 shells confirmed by Tony Williams in International Ammunition Association forum topic "Terminal ballistics" post of 1206 14 Apr 2014.








L28.^ This was suggested by numerous sources, firstly, per post 2 by LWD  on 04 Mar 2014 in Axis History Forum topic "Aircraft guns and bullet behavior".

L29.^ Berrigan, David, "They Smuggle Dope by the Ton", Saturday Evening Post, 05 May 1956, pp 157-158 (as referenced by McCoy, Alfred W, The Politics of Heroin (Chicago: Lawrence Hill, 2003), pp 183-184).












L30.^ Tony Williams post of 1206, 14 Apr 2014 in International Ammunition Association forum "Terminal Ballistics: ID of WW2 ammo holing a Thai bridge".