Ah, you certainly came to the right place. You are referring to one of Ambrose Bierce's greatest and most terrifying stories. It's called "One of the Missing," readily available on the Internet. In it, Private Jerome Searing, a Union soldier, is trapped and immobilized by a bomb shell in a collapsed barn with his own hair-trigger rifle pointed directly between his eyes. I wasn't aware it had ever been filmed, it is so deeply psychological, but it is truly frightening.
i read " occurrence at owl creek bridge " in jr high school, and then saw it on twilight zone. it made quite an impression on me. seeing " one of the missing " seems to have rekindled that interest. i recently saw it on public tv. here's the info http://www.aptonline.org/catalog.nsf/AlphaLookup/73078469D068559B85256E290074A2B4 thank you very much for the help.