The "Official Records" is a short title for a set of 128 volumes published by the War Department in the late 1800s and early 1900s. They contain many of the important reports on battles submitted by officers on both sides and a good sampling of what was called "correspondence"--letters written by various officers, orders of battles, general and special orders, and other documents. There is an index to the set that includes, among other things, all of the personal names mentioned. The reference I mentioned is the only one to McCormick. My "educated guess" is that the reference to him only made it into print because the rest of the message dealt with what was happening around Vicksburg at the time.
Cornell University has placed digitized versions of the set on their web site: