GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Whether it was 7.7mm or 13mm (or any types in between), they will make holes in a B-17 (or any other aircraft, for that matter) and holes are not a good thing: especially when there's crewmen, oil/fuel tanks, oxygen bottles, hydraulic lines, flight controls or engines in the bullet's path.Don't think the 2 12.7mm mgs of the Ki-43 would be that effective in shooting down recon B-17s.
A B-17 (or F-9) did not pack a great deal of armor and even a single 12.7mm penetrating can wreak havoc.