Hi Bob -
if you haven't obtained the service records of your ancestor in the 11th/17th Ark. Inf. yet, Bryan Howerton might be able to give you a capsule summary of his service?
Your ancestor is listed (as per URL below) as follows:
Cormac, G.L. ----Co. E 17th Ark. Inf. Pvt/Pvt
----Co. K, H 11th/17th Ark. Inf. Pvt/Pvt
Looks like he saw a lot of service from the beginning 'til the bitter end at Citronelle...!!!!