"Our command was soon afterward ordered to Port Gibson, to reenforce our troops engaged there in a bloody effort to repel the landing of the Federals; but the battle had disastrously terminated before we reached there.
"Pemberton forces retreated, passing around Vicksburg, until they crossed Big Black, and reached Champion Hills, near Edward's Depot on the Jackson road. At Champion Hills the Federals defeated Pemberton. Our brigade [Reynold's] lost no men, or very few, in this engagement, except those who were lost or captured on our forced march back to Vicksburg, as we were not actively engaged in the battle."
[Military Annals of Tennessee, Confederate, John Berrien Lindsley, Broadfoot Publishing Co., 1995, pp. 464/5]
As I thought, Robert was most likely conscripted [drafted] into the Confederate service which would explain his joining the Federal service. Here is some more information I picked up on Ahl's battery. Have not been able to ascertain the number of guns and calibers in the battery.
Official Records - Record of Events
Organized from Rebel prisoners of war who had been conscripted [drafted] into the Rebel service and principally northern men or Irish. The $25 bounty and $2 premium was entered on this roll as paid at the request of Major Henry B. Judd, United States Army, mustering officer.
From Wikipedia, the free encyclopedia
Ahl's Independent Company, Heavy Artillery (officially known as the 1st Delaware Heavy Artillery) was an heavy artillery battery that served in the Union army in the American Civil War. The company was mainly composed of former Confederate prisoners of war who had sworn allegiance to the Union (over 200 so-called "Galvanized yankees").
The company (Delaware's only heavy artillery company during the war) was organized at Fort Delaware on July 27, 1863, not long after the Battle of Gettysburg. The company was assigned to garrison and guard duty at Fort Delaware during their entire period of service. Its commander was Capt. George W. Ahl, and nearly all the officers had come to the fort with Independent Battery G, Pittsburgh Heavy Artillery. The company mustered out on July 25, 1865.
Ahl's Heavy Artillery Company
Another despised Union official came in the form of Captain George W. Ahl. Aside from his reported distaste for the Confederate prisoners, Ahl allegedly oversaw and/or instigated some of the abuse himself. Even worse was the unusual battery that Ahl commanded. The 1st Delaware Heavy Artillery was a battery in the Union army that was composed primarily of former Confederate prisoners of war. Those soldiers that could prove they were conscripted to serve in the Confederate army and were willing to take the Oath of Allegiance to the United States were allowed to join. Over 175 men took the offer and joined the battery. They were assigned garrison and guard duty at Fort Delaware during their entire period of service. Men that had been prisoners of war just days earlier were now tasked with overseeing their former companions. Needless to say, they were met with bitter contempt from the Confederate prisoners, but were also completely mistrusted by their new allies in the form of fellow Union soldiers. However, the men did ultimately receive better treatment and rations than their former inmates
Ahl's Battery, 1st Delaware Heavy Artillery (27 JUL 1863 - 25 JUL 1865)
This unit was formed at Fort Delaware and credited to the State of Delaware. Officers and non-commissioned officers for this unit came from Independent Battery G, Pennsylvania Volunteers while the bulk of the enlisted men were "galvanized Yankees" recruited from the Fort Delaware prison pen. The battery was officially mustered in on 27 JUL 1863 at Fort Delaware and mustered out at Fort Delaware on 25 JUL 1865. [http://www.fortdelaware.org/Union%20Units.htm]
Ahl, George W. Captain
Black, John S. 2nd Lieutenant
Campbell, Robert C. 2nd Lieutenant
Hay, Frank W. 2nd Lieutenant
Lewis, Robert H. 1st Lieutenant
Wolf, Abraham G. 1st Lieutenant
Fort Delaware Tour
Ahl’s Independent Battery – Questioned Loyalty
Meet the members of Ahl’s Independent Battery, 1st Delaware Heavy Artillery, organized at Fort Delaware from the Confederate POW ranks in the summer of 1863. Talk to the men who decided to change the color of their uniform – and allegiance – to guard their former comrades‑in‑arms. They have now been in the Union Army for a year, dealing with mistrust from both the Confederates and the Union soldiers. They are not living in the POW barracks any longer, but their lives have not gotten much more comfortable. Those in the battery who thought signing up with the Union would bring them freedom have learned that U.S. Army life is not an easy one. Some have even deserted, risking punishment of death should they be caught. This is a unique opportunity to learn about the Civil War from men who were in both armies.
“What’s for dinner?” you might ask. How about shepherd’s pie, with some fresh lemonade to wash it down? Compare “receipts” from 1864 to your recipes of today with Miss Susan Byrne, who’s been hired to cook for Captain Ahl and his family. Find out what the officers’ favorite dishes were.
Here's another who joined up:
Co. B—Private. Enlisted in Co. A, 17th Arkansas Infantry, December 2, 1861; transferred to Co. D, 21st Arkansas Infantry, May 15, 1862; transferred to Co. B, 12th Arkansas Battalion, June 30, 1862; present, February 28, 1863; captured on Big Black river, Mississippi, May 17, 1863; enlisted in Ahl’s Delaware Heavy Artillery at Fort Delaware, July 27, 1863.