Frank Leslie hired McCallum to add to the presence of Frank Leslie’s Illustrated Newspaper artists at the siege of Petersburg in the summer of 1864. McCallum arrived at Petersburg with E. F. Mullen, also a new Leslie’s artist, in late July just in time to witness the explosion of a mine placed in a tunnel under the Confederate lines. In spite of the explosion, the Union forces suffered terrible losses in their attack. McCallum recorded the extent of the carnage in often excruciating and grisly detail. On a lighter note, he did a series of drawings for Leslie’s on the varied and imaginative architecture of chimneys that soldiers built for their winter quarters. After the war, McCallum lived and raised a family in Washington DC. The Becker Collection contains drawings done by McCallum in Virginia between 1864 and 1865.