what would the ammo mix of 20mm guns carried by fighters have been? 

Point A looks like an AP round, probably 20mm.

points B and C look like they fit what is described as "Edge effect", rather than penetrating, it slid/yawed to the side.  probably 20mm, maybe even .50 caliber could do it.

does point D look to anyone else like an aphe or some such?  fairly clean impact with damage around it caused by fragmentation?    I remember the pictures that have been posted of HEAT impacts, with the scarring around them.   HEAT would not have been likely for this impact, but I would think APHE might have a similar scarring.   could it have been simple HE?

point E looks like it could possibly be a tumbling round hitting, without scale, hard to guess.  It might also have simply been an impact from something on a moving train, if I am interpreting its location right.

F makes me wonder.  20mm?   the diameter almost looks like it should be 25-30mm shell.   Could a Japanese aircraft have been firing at it, at some point?

G sounds like it should be a 40mm to 57 mm shell.   40mm S guns, okay.   otherwise?

Thailand has had a lot of post WW2 coups and a communist insurgency in the North in the 60's. Those holes could be the work of the Thais themselves

EwingGreg said...

what would the ammo mix of 20mm guns carried by fighters have been? 

 

Late-war RAF: 2x SAPI and 2x HEI

With regard to the suggestion that military coups and / or anti-Communist campaigns might have damaged the bridge: nothing is impossible; however, it's rather unlikely. I'm not aware of any rail structure having been damaged as a result of such activity. 

I'm currently trying to flesh in a hypothesis that an exploding HE shell might have produced a hole larger than the caliber of the shell, this to explain the 60mm hole diameter at Point G. It has now been suggested that this explanation might be irrelevant because 20mm HE shells may have had time-delay fuses, that delay being around one millisecond after impact. The delay was intended to allow a shell to penetrate an object before exploding which presumably would then have done more damage. That delay would also have been sufficient for a shell to have exited the built-up structural member entirely to explode harmlessly in an open area of the bridge.

Hence, the question: might WWII-vintage 20mm HE shells, as used in aircraft Hispano cannons, have had time-delay mechanisms?

One of the issues here is structural Iron and steel is not Armour. It is likely to be penetrated with a more ease and with more metal being carried away by the hit's.

The reasoning for thinking RAF is the geography, Northern Thailand in 44/45 is definitely RAF territory. There may have been some USAAF aircraft around but they will generally stop around .50, Maybe some P39's or P38's may have been in theater but most of the RAF stuff would be carrying 20mm at the high end with some specialist ground attack anti river and rail Hurricanes with 40mm.

What is needed is a nice online OOB for Allied forces in the Burma theater.

If it was my baht on the line I'd bet those holes wee the work of the Thais themselves. Probably some lovers tiff between some corrupt Thai air force boss vs his opposite Thai army crook. Back in the day it was probably a Sabre jet or F-5 that hosed down that bridge to teach the local Thai army boss to show respect. The Thais have a saying about Indians, translated from Thai to English it goes like this; "If you are walking through the jungle and come across an Indian and a snake kill the Indian first because you can trust the snake more." In my book same same the Thais.

Bear in mind that penetration holes could be bigger than the projectiles. I have a piece of armour plate on my desk with a 6mm diameter hole punched through it. The damage was done by the 4mm diameter tungsten core of a 6.5x25 CBJ.

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. 

You will never be able to determine the firing point from a single hole in the bridge.

I have some ballistic data for the 57mm tank gun from which trajectory curves might be calculated, but I'm not sure that would prove anything.

There was also an aircraft gun which fired the same basic ammo (same case, different loads) but hardly any of them (if any) saw service.