I've been looking at a railway bridge here in northwest Thailand which apparently has three shell holes from WW2.

I've completed a first draft for a webpage on the subject here: San Khayom Bridge. 

But my presentation is weak because I can't really say with any assurance what produced the damage. Since Thailand was a member of the Axis during WWII, I think I can reasonably assume that the two smaller holes probably came from an Allied aircraft strafing Thai railway rolling stock as it entered the bridge --- that is, if the projectiles that hit the bridge, were 50BMG which "spread" on hitting structural steel, growing from .50 caliber to 7/8 and 1 inch. A possible source of the larger hole with diameter over 2 inches, though, is less clear. While my suspicions are that Allied aircraft did the damage there, I'm hoping that someone here might have a feel for the behavior of bullets / projectiles when they hit / penetrate structural steel. And perhaps, but unlikely, an accident perhaps, an IJAAF aircraft might have been firing on the Allied aircraft pursuing the train and the bullets came from the IJAAF. 

So I'm looking for assistance / direction in interpreting the damage as pictured on the webpage: identifying ammunition and associated guns used, and types of aircraft associated with those guns --- and comments on the assumptions I've already made on the webpage, including my suggested scenario for Hole Nos. 1 and 2 of an aircraft pulling out of a dive while firing. 

I thank you for any assistance.

If it was strafed by American Aircraft, then the usual suspects would have possibly been .50 caliber mg, 20mm cannon, and maybe  37mm cannon or even 75mm cannon on a B-25H or B-25J 

There is in effect no real chance of a penetration of bridge steel by by either .303 or .30. The remaining allied calibers are as you say .50 ( mostly US ), 20mm ( mostly UK ), 37mm ( on P39 ), 40mm ( on some UK aircraft ) and occasional 75mm ( on a B25 ). There could of been the possibility of a rocket but they are all too big for the damage.

One thing I can tell you is that the damage was almost certainly not caused by the US navy or Marines they had only 117 combat flights in SEA during WW2 and dropped only 39 tons of ordinance. That doesn't really help you out much but I would think there should be some USAAF and RAF doco of missions somewhere.
 

Given the geographic location, I would suggest a attack by ground strike RAF Hurricane IID's with Vickers S 40mm may be the cause of the larger hole. These aircraft were active in the Burma/Thailand area post 43 as most were shifted east post the Desert war ending.

There is also an outside chance of a Chinese airstrike but good luck finding any documentation there.

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.