The first is the Chief Ordnance Officers request for resupply after the battle, which is a pretty fair indication of the amount expended (ref).
800,000 .57 rifle-musket cartridges (for Springfield and Enfield rifles*)
100,000 .69 rifle-musket cartridges (for M1842 converted to rifles)
200,000 .54 rifle-musket cartridges (for Lorenz rifles)
200,000 .69 musket cartridges
30,000 Sharp's cartridges
= 1.33m infantry and cavalry weapon cartridges
800,000 .57 rifle-musket cartridges (for Springfield and Enfield rifles*)
100,000 .69 rifle-musket cartridges (for M1842 converted to rifles)
200,000 .54 rifle-musket cartridges (for Lorenz rifles)
200,000 .69 musket cartridges
30,000 Sharp's cartridges
= 1.33m infantry and cavalry weapon cartridges
(Average per infantryman = 18.2 rounds expended, or 22.1 if 6th Corps excluded)
1st Army Corps Ordnance Officer reports his expenditure (including some issued to 3/11th Corps) in the OR (ref)
1st Army Corps Ordnance Officer reports his expenditure (including some issued to 3/11th Corps) in the OR (ref)
67,000 .58 rifle-musket cartridges
107,000 .57 rifle-musket cartridges
18,000 .54 rifle-musket cartridges (inc. 13,000 issued to 3rd Division, 11th Corps)
1,000 .69 cartridges (not stated whether they were rifle or smoothbore, but probably rifle, as it was issued to 3rd Division who had no smoothbore armed units)
34,000 cartridges of all natures by 2nd Division
= 241,000
Additionally the record shows that the Ordnance Officer issued 5,000 rounds of captured ammunition to 3rd Division.
Broken down as:
1st Division expended 80,000 rounds = 20.9 rounds per man
2nd Division expended 34,000 rounds = 10.9 rounds per man
3rd Division expended 119,000 rounds (adding the captured rounds) = 25.7 rounds per man
The third source is Geary's Report on 2nd Division, 12th Corps of 29th July (ref). He reports an expenditure of 277,000 rounds (69.4 per man), but it is "in the fight on 3rd July, and in the subsequent skirmishing;" (emphasis mine). This basically means it is for the campaign rather than just that day, although of course the bulk would be fired at Gettysburg. One assumes Geary took this figure from his Division Ordnance Officer, and one must make the assumption that all these rounds were issued to Geary's Division (if Williams' train wasn't present...).
The fact that Geary calls attention to this figure implies it is abnormally high. An examination of their situation on the night of the 2nd/3rd July shows they spent the whole night "in contact". After they stormed the works they simply laid down a continual barrage at the woods to their front, for some 7 hours. My copy of Gottfried's "Brigades of Gettysburg" references several primary accounts here which show Geary's (and Williams') situation was different to much of the rest of the field. Their skirmish lines fired continuously into a woods at an unseen foe.
Thus, taking the 12th Corps as an aberration,
1st Corps fired 228,000 rounds and was engaged on all three days. Geary's division expended upto 277,000 rounds on the 2nd/3rd. The rest of the infantry thus expended 795,000 rounds (18.2 rounds per man if 6th Corps fired none). This of course includes officers, sergeants and non-combatants. Deducting 35% for non-shooters (15% being officers and sergeants with the combat units and 20% being non-combatants) then roughly 38,169 riflemen were engaged (again, excluding 6th Corps) which pushes the ammunition expended by the average rifleman to 34.1 rounds.
In perspective, this is a fairly high ammunition expenditure. 20 rounds per man was the expectation of a typical battle at the time.
Edit: There is a fourth figure. The Berdan's Sharpshooters used 14,400 rounds from their Sharps in the 3 days. Their 450 riflemen give an average expenditure of 32 rounds per rifleman (ref). This is in line with the above estimates.
* As has been noted elsewhere, .58 ammunition was rarely issued for Springfields by this point. They used .57 because it was looser and the musket would foul less quickly.
