The histories of both the 51Va and Rives Battery (in the volume by W.C. Sherwood entitled The Nelson Artillery Lamkin and Rives Battery) in the Va. Regimental Histories Series list a John W. McGann, but attribute service in those units only to a postwar record. What that postwar record is is not clear. Furthermore, there are no wartime files that I could find that support his service in the 51Va or Rives' Battery. While it may be a long shot, you might want to look at, or have someone look at, the actual muster rolls from the 51Va to see if John W. is listed. Check with the National Archives to see what muster rolls of the 51 are available. I suspect the ones that might have anything about John W. would be those through April 1862. In my research I have found that sometimes the compilers of the Compiled Service Records have left out names that were on the actual muster roll.

As you note, John W.'s widow's pension application asserts he served in Rives Battery. However, when she lists his commanders she gives Wharton [Gabriel C.] and Dillard. Wharton was the first comander of the 51Va and remained in that position until promoted to brigadier general in 1863. Dillard was John Turner Dillard, who commanded the Nelson [County] Volunteers, E/51Va., from its initial enlistment in July 1861 until April 1862. When she gives names of comrade witnesses, she lists Robert Fitzerald and John Saunders. A Robert A. Fitzgerald is listed as a member of Rives' Battery, who was discharged for disability in November 1861. A John J. Saunders also served in Rives' battery from June 1864.

Rives' Battery, part of Walker's artillery battalion, had moved by noon 9 April 1865 to Red Oak Church, several miles NW of Appomatox C.H. There it disbanded. John W. is not listed among the Appomattox Paroles compiled by W.G. Nine and Ronald Wilson, H.E. Howard, Inc., 1989.

Archie R. Mcgann's pension application is at: http://image.lva.virginia.gov/CP/html/17241.html but it is unreadable on-line. You might contact the Library of Virginia to obtain a legible copy. Archie R. Magan has a compiled service record in the National Archives that lists him through December 1864 as a member of G/51Va Inf.