The siege of Vicksburg lasted 47 days. The Union forces had a clear view of the town established at the top of the hills above the Mississippi River. By Day 42 of the siege, the Federals knew the daily habits of the town folk. The town generally avoided church. Because, church services would expose folks to enemy shot and shell. But, on the 42nd day of the siege, June 28, 1863, the Catholics wanted to attend Mass. Vicksburg, being a busy river port, had a healthy Irish population.
For reasons unknown, perhaps simple boredom, the Federals trained a battery of Parrott guns on the church early that morning. The Parrott guns were the rifled cannon pieces, more accurate than the traditional guns. The Union forces may have simply observed an unusually large number of persons in the streets. In any event, they opened up on the congregation. Several persons were struck by shrapnel. No one was killed. But, Michael Donovan, an elderly and respected member of the city, sustained lacerations to his arms as he emerged from the church. A shell penetrated the church, but miraculously did not detonate.
A.A. Hoehling, Vicksburg: 47 Days of Siege (Penn.: Stackpole Books 1996), p. 237-238.