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.
Despite the awesome firepower, the guns were selected individually,
never in unison. I cannot recall a single instance in about 100 combat
mission where we fired more than one gun at a time, but we frequently
fired most of the guns during a mission. Our greatest limitation was
ammunition capacity. The AC-130E's that I flew could stay aloft for
more than 7 hours, but we could fire all of our ammo in a fraction of
that time, so we got very selective in how we used it. For example, we
rarely fired more than 3 or 4 rounds of 40MM at a time, same for the
105mm.
 
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