As a colonel on the staff of Maj. Gen. George B. McClellan, Hunt organized and trained the artillery reserve of the Army of the Potomac and fought with it in the Peninsula Campaign. Throughout the war he contributed more than any officer to the effective employment of the artillery arm. With the artillery reserve at the Battle of Malvern Hill, his 250 guns repelled repeated Confederate infantry assaults with such gruesome efficiency that the Union infantry had little to do. He personally commanded a group of 60 guns that he employed as if they were a single battery (usually Union batteries contained six guns).