Per Art Bergeron's "Guide to Louisiana Military Units, 1861 - 1865", the 6th LA. Infantry were driven off of Marye's Heights at Fredericksburg on May 3, 1863 and also fought the next day at Salem Church. There is a good book specifically on the 6th LA. called "Irish Rebels, Confederate Tigers: A History of the 6th Louisiana Volunteers" by James Gannon published in 1998. It gives a detailed history of their movements that you can use to track your ancestor.