A neat little book called "Sniper" published about 12 years ago (I think) talks us through the whole world history of the subject,bravely not glossing over the gory bits,hence we read of men with half their head blown off taking hours to die..
By the way,the well-known and much-hated Brit business tycoon Robert Maxwell was a sniper for a while during WW2,and I think the experience badly scarred him mentally for the rest of his life almost to the point of insanity,although he never admitted it.
He vanished overboard from his luxury yacht in the Atlantic,and although his naked body was later recovered,nobody is sure whether it was suicide or accident. Poor guy.