In early September, the two battalions were re-united under the command of McNeil, who had been sick; the Pennsylvania Reserve Division, now designated as the Third Division of the I Corps of the Army of the Potomac. The regiment was rearmed with Model 1859 Sharps breechloader rifles prior to setting off on the Maryland Campaign. At Bull Run, Kane was promoted to brigadier general for covering Pope's retreat. His position was filled by Edward Irvin. The new major was Alanson Niles. The regiment was decimated at Antietam, losing Colonel McNeill. The new commander was Charles Taylor, who had been captured twice by Confederates. They participated in the assault on Fredericksburg. Irwin resigned from injury, and Niles became lieutenant colonel. The Pennsylvania Reserves division was severely depleted from months of campaigning and in early 1863, they were sent back to Washington D.C. to rest and refit. The 13th Reserves thus did not participate in the Chancellorsville Campaign.