Huston had bought two M1903 Mannlicher Schoenauers (and cartridges) prior to the 'African Queen' shoot as he was eager to go Elephant hunting (though clueless about DG). It has been said that was why he hired on as director.
Perhaps it was loaded and the photog had just worked Huston's last nerve?
Bogey: "Now, you really oughtn't have said that."
Bacall: "We're going to need another still photographer."
Guy in background: "Oh, he'll be still enough in a moment."