Comments:
1) The MiG15 had a centrifugal comprerssor engine which is
considerably more resistant to battle damage tan an axial flow. Sveral
T33 (Allison J33 centrifugal engine) tow planes at Nellis took 50 cal
hits from clueless students getting sucked into deep six and still
firing at the banner tow target. This PO'd the tow pilot who would
jettison the banner into the shooter's face. But the T33 somehow
always got back home albeit with holes in the turbine disc. (Only two
guns were loaded in the 86s, and the students were poor shots.)
2) At high altitude hits in fuel tanks with 50 cal API did not result
in fires as would occur at lower altitudes.
3) Note that 50 cal API is considerably more effective than ball -
each strike results in a ball of fire about a foot in diameter. (Main
value in air to air for the shooter is that you can tell you're
getting hits).
4) Some of the 20m cannon were not very reliable - including the one
in the AD and the one in the F8. (OTH the M61 - although a different
breed of cat - is very reliable.)
5) The A10 was designed about a tank-killing gun - so the 50 vs 20
argument is not germane at all.

Walt BJ