I'm sorry I did not read your post close enough. What I have here says
Twenty Eighth Georgia organized as Siege Artillery After January 1864 serving as infantry
Mercer Artillery (See Company K 28th Battallion)
Company K Mercer Artillery of the 28th Lists
Captain A. J, Macarthy Appointed Nov. 2nd 1863
Lieut. John W. Flint Appointed Nov. 2nd 1863
2nd Lieut. A. N. Hines Appointed Nov. 2nd 1863
2nf Lieut. A. K Jennings Appointed Nov. 2nd 1863

From Olustee Website
http://extlab1.entnem.ufl.edu/olustee/28th_GA_art.html
During the Olustee Campaign the Twenty-eighth Battalion,Georgia served as infantry under the command of Major A. Bonaud. A detachment from the Florida Camp of Instruction, the Florida Conscript Company, was attached to the battalion at Olustee, and there is a slight possibility that part of the Second Florida Battalion was as well.

The battalion had been formed at Savannah in 1863 by uniting a number of independent heavy artillery companies. Olustee was the first major engagement for the unit. Comparatively small in numbers, the unit suffered over 100 casualties: 12 killed, 95 wounded, and two missing. The Florida Conscript Company suffered losses of six killed or died of wounds, and nine wounded. It is unclear if these loses were included in the totals for Bonaud's Battalion.

Two companies of this battalion served with the Army of Northern Virginia, with the rest of the battalion remaining in the Department of South Carolina, Georgia, and Florida

22nd Battalion Seige Artillery Company E Montgomery Guards Originally Lawrence J. Guilmartins Infantry Company Attached to 1st Vol. Ga Infantry
Captain Christopher Hussey Appointed October 9th 1862
Lieut. Thomas Houlihan Appointed October 9th 1862
2nd Lieut. Christopher Murphy Appointed October 9th 1862
2nd Lieut. Maurice Crowley Appointed October 9th 1862
2nd Lieut. B. M. Neely Appointed January 1st 1864
2nd Lieut. J. E. Holmes Appointed August 16th 1864

I hope this helps