You can read unit histories for Mississippi units here...

http://www.mississippiscv.org/mississippi_history.htm

McGavock Confederate Cemetery does have errors. The best way to find out if he transferred from the 5th to the 15th is to get his service records. You can order those from here off the message board. Just use the link at the top of this page. You can also order from the Mississippi Department of Archives and History but if you use our service it helps pay for these boards.