Complex sentence: The revolver that Hexum used was loaded with blanks and he apparently believed that the stunt was a harmless prank however the overpressure wave from the discharge of the blank propelled the round's wadding into his temple.
Split into simpler sentences: The revolver that Hexum used was loaded with blanks and he apparently believed that the stunt was a harmless prank. However, the overpressure wave from the discharge of the blank propelled the round's wadding into his temple.