Totally by chance, and only after I put this question to the forum, I finally came across a fairly detailed description of the strafing process on the Internet for the 13th Bomb Squadron website in its Combat Tactics
: Strafing (Area). The aircraft used was the Douglas A-26/B-26 Invader, apparently for combat in Korea:
". . . from a low level approach to the target. . . . a desired 320 mph. . . . At approximately 3,000 yards from the target, the pilot pulls up to 300 ft. above the terrain, spots his target, . . . At approximately 1200 yards from the target, the pilot enters a gentle dive, lines up the target in the sight and commences his strafing run. Firing bursts should be short, starting at 1,000 yards, and breaking off at 400 yards. Gently fan the rudder . . . to effectively cover the target area. . . . Airspeed . . . is the best defense against enemy ground fire while on a low-level strafing run. . . ."
Key factors in the description are a starting point, one, at an altitude of 300 feet (100 yards) and, two, at a distance of 1200 yards from a target. If the pilot flies a straight line toward his target, his angle of attack will be (rounded):
arcsin(-100 yards / 1200 yards) = -5°
The minus sign is a convention indicating he's flying downward, not upward. At the 400 yard breakaway point, he would have used up two-thirds of his 300 foot altitude; so he would be at 100 feet.
The conclusions to be drawn are: one, an angle of attack as shallow as -5°, and two, a breakaway altitude of 100 feet, are both realistic, especially during actual combat conditions. Moving into terminal ballistics, an assumption I think is reasonable is that a comparatively weak, thin walled
target, such as 10mm of structural steel will not much alter the course of a 20mm AP round; if true, then a projectile's exit angle from a target should be close to the angle of the source gun; in the case of an aircraft, that would be its angle of attack. As a consequence, the 12° exit angle measured at Hole A in the 10mm plate at the San Khayom Bridge indicates the bullet came from the air, not the ground. If the assumption is wrong and say instead that the 7° "floor" of the hole represents the impact angle, that 7° angle still does not reach the 5° angle calculated above, and so is still not in conflict with the conclusion that the projectile came from the air; ie, an aircraft.
Dramatic confirmation of the 13th Bomb Squadron's strafing instructions lies in the gun camera footage suggested by LWD. As he noted, "it wasn't unheard of for strafing planes to hit trees." From Aerial Dogfights and Strafing
, this sequence where a pilot parts the trees to finally come about level with a locomotive he's firing at (frames enhanced by IrfanView):
There is much more gun camera footage of low-altitude strafing, especially at 1945 New US Gun-Cam Footage from Germany!
: while the video has been dramatized with the addition of pilots' heads and unrelated frames of aircraft in general flight, the gun camera footage seems genuine.
Another item in the 13th Bomb Squadron instructions suggests the same source for Hole A could have caused Hole F: during the firing run, "Gently fan the rudder . . . to effectively cover the target area." But that only makes differentiating the P-38 from the Beaufighter more difficult; and I don't think that a harmonization chart for a Beaufighter will help in choosing the more likely attacker. The 13th's instruction is confirmed in an excerpt from the Military Channel at American warplanes strafing enemy trains during World War Two
at 04:08-04:19 and again at 04:40-04:59.
I guess my question now to the forum is: does anyone see any major problems in this tentative explanation?