Cub Driver wrote:

A normal bullet hole would be no problem. There's already a much
larger vent to the outside, which stabilizes cabin pressure against
the fresh & heated air being pumped in from the engines. People
smarter than I say that this hole is about three inches in diameter.



Dunno about smart but yes the 'hole' is indeed 3 or 4 inches wide
(and there's two usually) but they're not wide open all the time
(only when 'dump' is selected). They have another valve portion
which mates with them and regulates the 'outflow' to regulate the
cabin pressure which controls the 'cabin altitude'.

The pressurization regulator normally allows the cabin altitude
to slowly climb to roughly 8,000 - 9,000 feet while the a/c is
climbing to 35,000 - 40,000 feet. This puts the 'differential
pressure' (between the cabin and the outside pressure at roughly
8.5 PSID. (Pounds Per Square Inch Differential). This may not
sound like much but with a huge area such as a 747's cabin
there's a tremendous potential force there. That's why a small
explosion that rips a good sized hole in the skin can have
disastrous effect (No, a bullet hole is no problem as several
have mentioned)
--

-Gord.