Based on his age and time of enlistment, it would appear that he was enrolled under the Conscription act.
22nd Battalion, Georgia Heavy Artillery
22nd Artillery (Siege) Battalion was organized at Savannah, Georgia, during the summer of 1861 with six companies, increased to nine in the spring of 1864. Many of the men were from Burrow, Oglethorpe, Montgomery, and Cobb counties. The unit was attached to the Department of South Carolina, Georgia, and Florida, and served in the Georgia coast at Fort Pulaski and Fort McAllister. In 1865, operating as infantry in Elliot's Brigade, it saw action in the North Carolina Campaign and surrendered with the Army of Tennessee. The field officers were Lieutenant Colonel William R. Prichard, and Majors J.B. Gallie, Lawrence J. Guilmartin, and Mark J. McMullan.
Predecessor unit:
Burtwell's-Girardey's-Pritchard's Battery was organized at Athens, Georgia, in April, 1861, with men from Clarke County. It participated in the fight at Shiloh, then was assigned to J.K. Jackson's Brigade and fought with the Army of Tennessee from Murfreesboro to Missionary Ridge. The company was later converted to heavy artillery and attached to the 22nd Georgia Artillery Battalion. Its captains were John R.B.Burtwell, Isadore P. Girardey, and W.R. Pritchard.
http://www.civilwar.nps.gov/cwss/regiments.cfm
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22nd Siege GA Heavy Artillery Battalion
Organized: on 11/26/62 - Mustered Out: 4/26/65
John G. Rice
Enlisted as a Private
"E" Co. GA 22nd Siege Heavy Artillery
Sources used by Historical Data Systems, Inc.:
- Index to Compiled Confederate Military Service Records
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John G. Rice, Private, Company E (Montgomery Guards), 22nd Battalion Georgia Artillery, enlisted August 22, 1863 at Macon, Georgia by Lt. Col. Harris for 3 years or war, captured at Savannah, December 21, 1864, admitted December 24, 1864 USA 1st Division Hospital, Chronic Rheumatism, age 46, arrived Fort Delaware, Del. prison camp March 12, 1865, died June 16, 1865, Diptheria
M266: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Georgia
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See also:
http://baca22ga.tripod.com/index.html
http://www.rootsweb.ancestry.com/~gatroup2/22ndbattalionheavy1.htm
http://www.ranger95.com/civil_war/georgia/artillery/22nd_%20bn_hvy_arty.html