Punctured pressure cabin.

"Cub Driver" wrote...

And more detail yet. This too from rec.aviation.piloting:

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Anyone
familiar with the structure of an A/C will immediately ROFL at the
idea of a 9mm bullet penetrating the external skin if fired from
inside the cabin.

It would take a substantially more powerful weapon than a 9mm to cause
a "Window Failure",
.. . .

The same applies to a bullet exiting through the A/C skin. Consider
between what is seen as the interior cabin wall & the "External Skin"
of the A/C is a layer of insulation, assorted wiring, plumbing in
some places, plus untold ribs, stiffeners, & other assorted structural
components all of which have some "Curvature" to them.
.. . .

There are a few places a "Very High Velocity Bullet" of large caliber
could possibly exit the external skin if it the internal point of
impact was at a "very specific angle, very close to 90 degrees to
external skin" if fired from close range internally.
.. . .

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(Again: the above is quoted from rec.aviation.piloting)

....which doesn't mean anything as far as credibility goes! Who wrote that?!? I
am almost ROTFL at some of the assertions made up there (salient parts
retained)!

Though the plexiglass in the cabin windows is tough, it isn't THAT tough! A 9mm
or .40 S&W round would EASILY penetrate both layers! The flattened edge of a
JHP round would help prevent ricochet when it hit the thin inside layer, and
mushrooming would probably start without any substantial decrease in velocity.
The net result would be a hole of about 1/2" in diameter. Whether or not the
window would fail immediately or eventually is a matter of conjecture, but is
entirely within the realm of possibility.

As for the aluminum skin, it is already stressed by the differential pressure,
and could not resist a similar round. The interior trim and insulation would,
again, start the JHP mushrooming, and maybe slow it down somewhat, but not
enough to prevent it from penetrating the thin aluminum skin.

IF the bullet happened to hit a rib, it may well be stopped. An overlapped skin
section would not stop it, however. Air ducts wouldn't affect it much at all;
hydraulic lines would deflect it rather than stop it or cause it to "ricochet";
and a THICK wire bundle MIGHT slow it enough to prevent penetration.

---------------------
John Weiss
Retired Naval Aviator
current 747-400 pilot