Barclay and his brother Edwin Coppoc initially met John Brown in Springdale, Iowa, and joined Brown's army in Chambersburg in summer 1859. Barclay, Owen Brown, and Francis Jackson Meriam stayed behind at the Kennedy Farm during the raid to guard weapons. When the raid failed, they escaped to Ohio. Barclay Coppock actually made it all the way to Canada before returning to Ohio. He and others spoke at a meeting the day Albert Hazlett and Aaron D. Stevens were executed. During the Civil War, Barclay joined the Union Army and was killed when Confederates sabotaged some railroad tracks and caused his train to crash.