Capt. Patrick H. White Post War Memoir. There's a copy on file at Vicksburg NMP, and its quoted as well in William's Chicago's Battery Boys. I don't have it handy, but I believe Ed Bearss says something similar in The Campaign for Vicksburg.
Here's the excerpt from White's Memoir:
"We used shrapnel, the fuse cut so close the shell exploded almost as soon as it left the guns. The first discharge was simultaneous with the rebels and struck their gun in the muzzle, scattering death among their cannoniers, I never saw a gun loaded and fired so quick as every man was at his best. They did not take much care in sponging and once or twice it went off prematurely."