This article is from the Vietnam FAQ, by Brian Ross, John R. Tegtmeier, Edwin E. Moise, Frank Vaughan, John Tegtmeier with numerous contributions by others.
Operating altitudes were, at the time, highly classified. They may
even still be classified, so I will be circumspect. Face it, after a
certain altitude, 7.62mm and 20mm will tumble. But, 40mm and 105mm
don't tumble. Needless to say, we gunners used a lot of yellow walk
around oxygen bottles.
The operating altitudes were letter-coded, alpha thru ?, referring to
AGL: alpha was 2500, bravo 3500, etc. AC-119K's usually operated at
delta or echo over the trail, and the AC-130s (as I recall) at echo or
one notch higher. The 7.62 were ineffective above 3500 AGL, so we
never used them along the Trail; the 20mm were good to about 5500 AGL,
as I recall. [RH]
However my damaged brain cells seem to recall that we flew at much
higher altitudes. I think that the "A" model AC-130s worked at the
altitudes you mentioned, and when we worked in SVN and Cambodia, we
worked lower, but we routinely had to dodge 57MM, occasionally 85MM
(?) and rarely 101MM (?) [it is hell getting old, I used to know those
altitude numbers like the back of my hand]. I do know that on a lot of
missions we absolutely had to use oxygen, and we used to wear thermal
underwear and arctic flight suits in order to keep warm. In fact, it
was quite funny right after takeoff. Most of the gunners carried a
large bag with them, and as soon as we were wheels up and it was safe
to walk around, we used to peel off our green nomex flight suits, pull
on our "long johns", put on our winter flight suit, put on a flight
jacket, wrap our pistol belts around our waists, put on our survival
vests and then put on our parachute harnesses, helmets and gloves. We
sorta looked like a huge green Pillsbury dough boys.
 
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