This is slightly OT, but as it's a commonly used comparitor (not least in Nosworthy's The Bloody Crucible of Courage) I thought it was appropriate to put here. Sorry for the digression into the Napoleonic War.
A commonly quoted figure of musket rounds fired per hit achieved is 1 in 459. The source of this figure is Henesay (original source). However his figures do not add up. For a start, only 3 British Divisions were heavily engaged at Vittoria (2nd, 3rd and 5th); only 10 British and 3 Portuguese regiments, and the Royal French Chasseurs Brittanique suffered more than 100 casualties. The 3 heavily engaged divisions had marching states of ca. 25,000 Officers and Present Under Arms on their state of 25th May, and had been marching and skirmishing for nearly a month before the main event. Henesay's count of 50,000 engaged infantry is highly unlikely. Also, the nature of the fighting at Vittoria precluded many standup musketry duels. While some may have occurred at river crossings, most of the real fighting was close quarter bayonet work, especially for the 3rd Division.
So, if Hennesay's estimate of over 3.6 million rounds fired is inaccurate, do we know how many rounds were fired? Yes, Henesay himself gives that information.
The way ammunition resupply worked on the Peninsula was that boxes of ball-cartridge were dropped by the Commissary behind companies, who would then send small details back to open them and distribute the rounds, the soldiers cartridge boxes were kept constantly full. Henesay's figures indicate that 1.35 million rounds were thus distributed. However Henesay notes only roughly half of these were fired. Thus we arrive at the approximate number of rounds the British actually fired, about 675,000, which is well within the norms for European combat (which reckoned on 20 rounds per infantryman being used in a battle).
Henesay's estimate of casualties is too high. The French Armies suffered 5,210 killed and wounded, exclusive of much of their artillery (no returns except for the Army of the Centre, whose artillery was overrun at bayonet-point) or the attached Spanish Royal Guards or elements of the Army of the North. Oman estimates (from pro-rating officer casualties, which were recorded) hundreds of extra casualties, probably, in my opinion between 5,700 and 5,900.
The proportion of these hit by artillery will never be known, but the artillery played little role in the British attacks, since the infantry kept outrunning their artllery support. If we make the assumption that no-one was hit by artillery then the proportions of musket rounds expended per hit is about 1 per 116. This is a high estimate, and in all probability the real ratio is in the 1 in 150 to 1 in 200 range common in American Civil War firefights.