See: http://www.tngennet.org/civilwar/csainf/csa39m.html

Robert M. Morrow, enlisted as a Private in Company C, 31st Tennessee Infantry on October 25, 1862 in McMinn County, Tennessee, captured at the Battle of Big Black River/Champion Hill, May 16/17th, 1863, sent to Fort Delaware via Camp Morton, Indiana, joined in U. S. Service August 1, 1863 into Captain Ahl's Battery

The regiments designation was changed to the 39th, the end of June 1863.

Robert Morrow enlisted as a Private "Ahl's" Co. DE Heavy Artillery, U. S. A.

Ahl's DE Heavy Artillery Company
( 3-years )

Organized: Fort Delaware, DE on 7/27/63
Mustered Out: 7/25/65

Officers Killed or Mortally Wounded: 0
Officers Died of Disease, Accidents, etc.: 0
Enlisted Men Killed or Mortally Wounded: 0
Enlisted Men Died of Disease, Accidents, etc.: 11
(Source: Fox, Regimental Losses)

Ahl's Heavy Artillery Company
DELAWARE

Organized at Fort Delaware July 27, 1863, and garrison duty at Fort Delaware during entire service. Mustered out July 25, 1865.

Frederick A. Dyer "A Compendium of the War of the Rebellion" vol. 3

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Ahl's Heavy Artillery. -- Capt., George W. Ahl, First Lieuts., Robert H. Lewis, Abraham G. Wolf, Second Lieuts., Frank W. Hay, John S. Black, Robert C. Campbell.

This company embraced a battery at Fort Delaware, the members being ex-Confederates who had taken the oath of allegiance to the Union. The service was faithfully performed until the muster out of the men, July 25, 1865. Nearly the entire company was composed of those who had been taken to Fort Delaware as prisoners.

Source: The Union Army, vol 1, p. 526

You may procure his service records files at: http://history-sites.com/research/index.html