Automatic Spear Gun

He remembered what Val had taught them, so he had loaded the gun with six compressed-air darts and had set the trigger to automatic, which meant as soon as he pulled it, the six darts would be fired, one after the other, in a matter of seconds.

[...]

When he cleared the opening, all Wash could see was the massive body and the huge staring eyes of the awful, misshapen beast. The water had filled with writhing tentacles that all began to move toward him. He saw that he was underneath, and he saw the huge mouth that would tear him to pieces. It opened wide to receive him, and Wash now knew what death was.

Other tentacles began to enclose his arms and his legs.

Wash raised the dart gun, pointed it straight at the soft underbelly of the beast and, with a prayer to Goel, pulled the trigger. The gun bucked in his hand, but he held it steady.

Then he saw the darts—all six of them—penetrate the body of the octopus and disappear, buried deep. There was a series of muffled sounds as the expanding air exploded and literally blew the monster to pieces. For one moment, the tentacles pulled at him, probably a reflex action, and then they grew limp. Instantly Wash kicked himself free and back into the cave, his heart beating like a trip-hammer.