I too had a "dead end" with regard to an ancestor who served in company G, 42nd Ga. Vol. Inf., Private Robert W. Miller. His compiled service records gave very little info. besides what took place while the 42nd Ga. was involved with the Vicksburg campaign.

As pointed out by the previous two gentlemen, I would strongly suggest that you conduct a search for a pension application. When I discovered my ancestor's pension documents, they fully outlined his war time service, including incidents that took place well beyond the Vicksburg campaign. For instance we discovered that he had been wounded at Nashville, which was something that no one within the family knew.

Good luck!