Mismount

One afternoon his foreman, Joe, passes John's area just in time to see him mismount a piece on dures.

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“You can't afford to mismount a tire,” he said.

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If the gun is mismounted then the eye-barrel relationship is off and the benefits of the gunfit exponentially decrease the farther away the bird is.

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She was very stiff and lame, and was made more so by a mismount, catching in her saddle and tipping down on the bruised side; but she picked up and went at it again with a will, and after a wobble or two across the road, was bowling along at the same risky pace as before.

Source: wiktionary