Terminal ballistics: ID of WW2 amm holing a Thai railway bridge

Ad: This forum contains affiliate links to products on Amazon and eBay. More information in Terms and rules

I'm researching a railway bridge here in northwest Thailand which apparently has three shell holes from WW2.

I've drafted a webpage describing the situation here: San Khayom Bridge.

I'm looking for guidance regarding what ammunition and guns might have produced the holes, and it was suggested that I query this forum.

I thank you.
That's pretty interesting (and welcome aboard)!

I do have one question, assuming basic maintenance of the bridge would include regular painting, would not a lot of the detail of the holes have disappeared after 70+ years?
Granted. And each cycle of repainting was probably preceded by sandblasting. If I correctly recall a stencil on a beam at one abutment, this bridge was last painted 10 years ago. I don't know what the length of a repaint cycle would be --- it's probably gotten longer as paints have improved, and aside from a few "weeping" rivets, the paint right now appears to be in good condition.

You can see chips in the paint on the edge of the hole, to the left and to the right, in the closeup of Point F. That it is chipped is curious, for the hole is distant from the walkway.

I could go in with paint remover and take the coatings down to bare metal to see what detail has not been lost, but just obscured: however I'm sure that rail management would not be sympathetic. I was waved away from one station house where I simply wanted to take a photo of mechanical switch controls with their beautifully polished brass ID plates.

Life is full of compromises.
What about getting a metal detector and surveying the area around the bridge- say 100 meters square? You might recover some of the projectiles intact.
Metal detectors may be an option.

In the meantime, after substantial interruption, I'm back to the shell holes in the San Khayom bridge. I apologize for the length of this; but I don't know how to shorten it further.

With regard to the hole at Point A, I'm trying to visualize the actions of the aircraft that produced the hole. I'd like to include a sketch on my webpage of the possible / probable path of the aircraft during its attack.

The consensus is that the hole is apparently from a 20mm projectile; and in the Southeast Asia Theater both the P-38 and the Bristol Beaufighter carried HS.404 cannon which used that ammunition. I'll use the P-38 as a subject here simply because I can find more information about it (the Beaufighter might be the better candidate because, beyond its harmonization range, it would have offered a dispersion pattern with its four cannon: such a pattern might more easily have included Point F, the other 20mm impact point on the bridge. Does anyone know of an on-line presentation, including sketches, perhaps a manual, for harmonizing the guns on a Beaufighter?).

So, scenario: a P-38 pilot happened upon a "target of opportunity", a train traveling south out of Lamphun and approaching San Khayom bridge. The pilot reduced altitude so as to fire on the train. Coming into range, he fired for a certain period; then he broke contact and climbed away from the train and the bridge.

Angle of attack: The exit angle of Hole A measures about 12° to the horizontal. In the sketch below, I assume the bullet path was almost unaffected in passing through the comparatively thin bridge plate (projectile diameter was twice the thickness of the target plate). On that basis, I assume the pilot dropped to an altitude matching the 12° allowing an angle of attack of 12°.

A P-38's single 20mm cannon was centrally mounted in the fuselage and its operation was basically point (the plane)-and-shoot. The range of the cannon was limited primarily by the pilot's visual acuity and his properly adjusting for bullet drop (excluding weather, aircraft performance, pilot skills, whatever).

I assume an attack speed of 350 mph: a P-38's cruising speed was 275 mph and max speed was 414 mph. A velocity diagram would look like this for a 12° dive at 350 mph:


The important information here is that, flying at a down-angle of 12° and at a speed of 350mph, the aircraft would have been dropping 107 feet per second.

The idealized flight path would have looked like this:


Of course, the pilot would have disengaged at some point before his aircraft flew into the target.


Do these assumptions and the scenario seem plausible? If not, I'd appreciate being corrected.

If the scenario is not too far off:

  • 1. At what range would the pilot likely have started firing? (I read that one B-25 gunner had his guns harmonized for 1000 yards: he cautioned that this was useful only for stationary targets); and
    2. At what altitude would the pilot have had to disengage (how much altitude would be burned up after the pilot tried to gain altitude; and how much spare altitude should he have allowed)?
    Tying these down would also establish how long the pilot might have fired.
Alternate scenario: With the above presented, there is a piece of data which could contradict this scenario, and I don't have the background to interpret it: the "floor" of Hole A is at an angle of about 7° to the horizontal. If 7° (not the 12° used above) were the angle at which the projectile entered the plate, and the exit angle was, as actually measured, 12°, then passing through the plate would have bent the projectile's course by 12° - 7° = 5° (see cross-section).

If the angle of attack were a very lean 7°, then at a range of 1000 yards, the gun would have been only 366 feet off the ground --- with bullet drop compensated for by either, an extra 24 more feet for a total of 390 feet, or a slight tilt upward in angle of attack. And at 500 yards 183 feet (compensated for 4 feet of bullet drop). On the other hand, the aircraft's altitude loss while changing from a down-angle of 7° to actually moving upward would have been less than recovering from a down-angle of 12°. But operating that close to the ground at 350 mph or more seems unlikely (which is why I didn't include it in my main presentation above); though I am willing to be corrected.

Are there any comments about this alternative?
Thats some great detail there and on your website, not being an expert on ballistics etc I can't advise but I'm sure someone will.
... such a pattern might more easily have included Point F, the other 20mm impact point on the bridge.
More easily, yes, but a single gun is always possible. Dispersion and wander of aim in a long burst can cover a large area.

Does anyone know of an on-line presentation, including sketches, perhaps a manual, for harmonizing the guns on a Beaufighter?.

Unfortunately, with regards to the attempt in piecing together the exact strafing attack, I think this is impossible to do with any degree of certainty. There are just too many variables that are completely left up to the whim of the pilot.
Interesting, but you also have to keep in mind that there were quite a few Air Force bases in Thailand during Vietnam as well. And with the number of clandestine things going on during that time, there is the possibility that it could have been from that time as well.
Herman1rg: thank you for the compliment. My feeling is, the more data, the more accurate the conclusions. But, granted, too much data can obscure matters.

Greyman: as you conclude, piecing the actual attack is impossible with any certainty. Yes, most certainly; but I would like to present at least one realistic scenario to confirm plausibility. This started with uncertainty about whether the bullets had come from air or ground because of Hole A's very small angle with the horizontal. And below I hope I have found one plausible explanation for what happened.

Thank you for the Beaufighter cannon configuration. I'll play with that.

Evangilder: Yes, the dates for the holes will probably forever remain unresolved, so they could well have been punched during one of the more turbulent periods in Thailand after WWII.

Okay, where am I? Totally by chance, and only after I put this question to the forum, I finally came across this description of a strafing process on the 13th Bomb Squadron website:
View: https://www.youtube.com/watch?v=PLxI6kW7bFU, there is a sequence where a pilot parts trees to finally come about level with a locomotive he's firing on (frames enhanced by IrfanView):


There is much more gun camera footage of low-altitude strafing, especially at
View: https://www.youtube.com/watch?v=kYTaddev6KQ at 04:08-04:19 and again at 04:40-04:59.

I guess my question now to the forum is: with due regard for Greyman's and evangilder's caveats, does this tentative explanation otherwise stick together?
You have certainly formed a good analysis of your theory. The documentary evidence is good as well. It certainly makes sense that this is what it could be. I don;t suppose there are any people in the area that were alive during the 1940s who might have recollection of any strafing runs, are there? Or maybe some old news clippings or other written records? Just some things to consider to help bolster your theory.
Just curious, but why is it assumed to be a P-38 that attacked the bridge? The RAF and RAAF operated 20mm equipped aircraft in the area.

The Bell P-39 export had a hispano 20mm installed instead of the 37mm nose weapon and a series of the Hurricane was equipped with 20mm.
Evangilder: thank you. Survivors: just one, and curiously he speaks some English, and he is still lucid, or apparently so; however, he had no information. Written records: the big barrier, of course, is a different language with an alphabet unrelated to the English alphabet. Newspaper morgues don't seem to exist. The local office of the national archive behaves like a protector of priceless relics and gave the impression that my mere presence was disrespectful; that said, it was almost ten years ago that I last approached that self-important lot --- perhaps I should try again.

vikingBerserker: thank you.

GrauGeist: reasonable question. Greyman correctly responded. That's why Greyman provided a gun cannon diagram for the Beaufighter yesterday.
Gotcha...I missed that link while reading the rest.

Good research, though I noticed in the "Weaponry and platforms" tables (towards the beginning), it has the Brewster F2A listed under "British mfgr". They were Manufactured in New York for export as a "de-navalized" F2A, designated the B-339 for the Belgians (which the British acquired 32) and designated the B-339E for the British under the name "Brewster Buffalo Mk I", which they received 170.
For the 60mm hole at Point G, it has been suggested that a Bell P-39 Airacobra with its 37mm M4 cannon and an HE (high explosive) shell might have been the source. I've found no record of any unit having used this aircraft over Thailand.

What about the 40-mm Vickers 'S' gun used by Hurricane IId and IV aircraft?

In your writeup on the weapon you use two quotes;

'. . . In 1944, the aircraft served in the Far East, mainly firing HE ammunition against road and river transports. The units to which the specially armed Hurricanes were assigned are not identified.'

20 Squadron, 28 Squadron and 42 Squadron RAF used these aircraft. There may be more units but I couldn't see any more in my quick look.

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

I can't speak to any numbers/ratio used, but AP and SAP/HEI rounds were used as well. I know early on HE rounds were unavailable and the Hurricane units in the Far East were begging for them, as the AP rounds weren't ideal for basically any of targets they faced.

EDIT: added a bit
Last edited:
With regard to Greyman's last post, I've checked Shores and googled the Internet. Shores is the only significant source I found. With regard to both quotes, 20 Squadron appears to have been the subject.

Shores' Vol 3, p 214: [28 Apr 44] "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."

Shores, p 239: [June 44] [20 Squadron] "claimed the destruction of 12 tanks, 501 sampans, 336 dirsties, 348 dugout canoes . . . . by the end of the June."

Information about operations of 28 and 42 Squadrons doesn't seem as detailed (which is not to say that I didn't miss something in groping through Shores' fine print).

In any case, none of the three units appear to have operated over Thailand.

While I'm no ballistics expert, I'm leery about relying on the exploding of a smaller caliber HE shell (in this case a 40mm) to explain the 60mm hole. Were such to have happened, I should think that the resulting "petals" would have been bent back by the force of the explosion; but the petals as viewed from the side are quite uniformly perpendicular to the plate (see illustration).

I have double checked with a local military historian and he is unaware of any 57mm (or near) gun in the Royal Thai Army WWII arsenal. Calibers either side of that dimension are 40mm and 75mm.

Someone else has suggested that I look at the possible composition of the task force that the IJA set up in Thailand for, amongst other things, constructing a supply road from Chiang Mai to Toungoo (which failed) and also protecting the existing supply rail/road line Bangkok to Imphal. The task force was a very flexible entity according to Allied handbooks. It is conceivable that there was a medium tank unit in it, and the right model of tank would have had a 57mm gun. An enterprising young Thai enlisted man using an anti-tank gun shot down a B-25 during a bombing run on Kaeng Luang Bridge in Nov 1944; there's no reason that a tank couldn't plunk at a P-38 or Beaufighter flying at low level. In this case, the tank gunner would not have been so lucky.
I'm in the process of writing up what all I have to date on the two 20mm shell holes in the San Khayom bridge. I still have to deal with the 57mm shell hole (Point G).

I've not found as yet any aircraft, either Allied or IJAAF, operating over Thailand that used a 57mm cannon. On the ground, the Japanese had a Type 97 57mm Tank Gun, used in a Type 97 Medium Tank "Chi-Ha". The gun would have been firing at the RAF Beaufighter or USAAF P-38 which was putting the 20mm holes in Points A and F of the bridge. The scenario in itself is not farfetched: 170 km SE at the Kaeng Luang Bridge, an enterprising Thai gunner using an anti-tank gun shot down a B-25. But at the San Khayom bridge, no Allied plane was downed though it might have been targeted.

I've found no record of tanks with 57mm guns or independently-mounted (jury-rigged) 57mm tank guns having been assigned or used at Lamphun; however, I see two possibilities by which such a tank or tank gun might have been present to defend the Lamphun bridge (plus a longer bridge just 6 km north):

1. One of the main IJA supply routes for Burma followed the railroad from Bangkok to Lampang, 80 km south of San Khayom, where goods were transferred to road vehicles for transport to Kengtung, on to Mandalay, and then in 1944 to Imphal/Kohima, etc. There is also a story that an alternate supply route continued north by rail to Chiang Mai where goods were transferred for transport north into Burma. In both cases, rail transport came under heavy attack by Allied aircraft starting in early 1944 (there doesn't seem to have been much recorded of attacks on road convoys). I'm guessing that those Allied air attacks might have damaged one or more tanks or their carriers sufficiently that they were abandoned along the way. Afterwards, enterprising Thais or Japanese might have salvaged a tank, or a tank gun, for anti-aircraft defense at Lamphun.
2. In preparation for defending Thailand very near the end of the war, various IJA units were assigned to the general Chiang Mai area to meet any Allied ground attack from the north. IJA units included elements of the 4th and the 56th Divisions as part of the 15th Army which was itself headquartered in Lampang. All were essentially in position by June 1945. These units might somehow have acquired some Type 97 Medium Tanks, or at the least a Type 97 57mm Tank Gun, which found its way to the Lamphun area.

While this is wholly speculative, I do have the hole in the bridge which records the diameter of the projectile, its angle of impact, and its bearing. If I could get a set of ballistics curves for the tank gun, I might be able to estimate the location of the gun, if it had been fired from the ground, and check that ground for any possible evidence (that's where the metal detector will be a necessity).

So I ask the forum, does anyone have or know how to get ballistics curves for this Type 97, 57mm tank gun?

I thank you.

Users who are viewing this thread