Sentence:  How quick the eye and hand to catch him [the ruffed grouse] when he rises from the underbrush and is out of sight in the wood before the untrained sportsman stops him with what is little more than a <WSD>snapshot</WSD>, so instantaneously must all be done! 
Meaning of the word marked with <WSD> tags:  A quick offhand shot, made without deliberately taking aim over the sights.