Gettysburg is an interesting case. We have the resupply requests, and a few statements from ordnance officers are in the OR, and we know that
this is roughly what was expended:
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 (of these, Berdan's sharpshooters recorded they expended 14,400 rounds, and so the remainder is likely the cavalry)
= 1.33m infantry and cavalry weapon cartridges
The artillery expended 32,781 rounds (this includes a significant number of unfired rounds lost in caisson explosions etc.). The rebels had 24,200 casualties by the latest research by Busey and Busey, of which 4,890 were unwounded captured, leaving 19,310 men who were hit (including be melee weapons, a very small percentage which we'll ignore).
If all casualties were by small arms, then the effectiveness would be 70 rounds per casualty.
Of the hit, 3,446 were "killed", and the remaining 15,864 were various degrees of wounded (2,004 classed as mortal). Thus ca. 18% of these hit were killed outright, as you'd expect from artillery hits by solid shot. Of the corps which broke down by ammunition nature, only 11.6% of expenditure was solid shot. These are corps account for roughly half the expenditure, and was only about 3,800 solid shot were expended. The ratios indicate a lot of the artillery casualties must have been from shrapnel
Comparing to Inkerman, the Federals expended ca. 15 times the artillery ammunition and ca. 6 times the small arms ammunition for less than twice the number of hits.
Now, had either side been well trained in rifle shooting, it would likely have been decisive.