I looked in Booth's Records of Louisiana Confederate Soldiers and Commands and it lists a Pvt. Peter E. McKittrick of the Pointe Coupee Artillery surrenderimg at Vicksburg, so you can rule him out as the victim at Ft. Pemberton (also backed up by the gravestone). I agree that Benjamin McKittrick record ends when he is exchanged at Vicksburg on Sept. 20, 1862 BUT I highly doubt that he is the one killed. He is listed as a commissary sergeant which is not someone found working near a gun. Yes, technically it is possible, but unlikely. I would suggest checking the actual National Archives microfilmed (Series 320) service records of the Pointe Coupee Artillery which are in the Louisiana State Archives in Baton Rouge and several university libraries around that state like LSU-A in Alexandria. That is your best chance of not only ascertaining who it was but possibly the wounded fellow's names as well.