GETTYSBURG, Pa. — Like thousands of other re-enactors, Eric Mueller honors the sacrifices of soldiers in the Civil War by going to great lengths to live as they did — sleeping beneath a canvas sheet suspended on wooden posts, eating hardtack and salt pork, carrying 60 rounds of ammunition in a cartridge box and a backpack, and marching long distances in heavy woolen tunics.

But in the interests of safety and perhaps a little comfort, Mr. Mueller, 40, allows modest divergences from the 19th-century soldier’s life.

Last week, for example, Mr. Mueller packed in his knapsack two sweet potatoes and two small onions, foods that he conceded may not have been in season in southern Pennsylvania in the summer of 1863, and so would not have been available to Civil War troops even if they had tried to forage them from nearby farms.

Still, he subjected himself to discomforts like not washing for a week and squeezing his six-foot frame into a 5-foot-8-inch-long tent that he shared with another re-enactor. Mr. Mueller, a civil servant from Hawaii, said he stayed “reasonably dry” during four nights of camping out on Cemetery Ridge in the heart of the Gettysburg battlefield....