This is intended for people interested in the subject of military guns and their ammunition, with emphasis on automatic weapons.
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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):
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.
Those posts answer the question, I guess. I thank you: that'll save endless stewing.
I'll just present the information I've accumulated and leave it as another 'imponderable'.
I've attempted to finalize my webpages on this subject, but am of course still interested in feedback. There are some topics which I leave open: some seem more effort than is warranted; others can have no answers; a metal detector is on order as a long shot.
I appreciate the direction I've received from various contributors on this forum.
My treatment of this subject has grown to 11 webpages (which start here), and offers more detail than the average reader probably cares to wade through. As a result, I've started the subject off with a single page condensation --- it's a long page, but just one.
I know this may be a little simplistic, but have you asked the people in the local area? It's possible that you might even find an eye witness allowing you to determine when and, possibly, what. I've found the typical Thai to be a friendly, intelligent person willing to help when shown just a little respect.
I'll give you holes! How many? How big?