So I have some interesting numbers from here: http://www.napolun.com/mirror/napoleonistyka.atspace.com/infantry_tactics_2.htm

• According to R. Henegan the British infantry at Vittoria fired on average 459 rounds for 1 French casualty. (Henegan - "Seven Years' Campaigning in the Peninsula and the Netherlands", pp 344-345)
• Napier witnessed volleys fired by British infantry (in Spain) where out of 300 musketballs fired none hit the target.
• At Vittoria, the British infantry had on average 1 hit in 459 shots fired. I assume that the ratio for French infantry was lower, as they had much less training.
• Hughes calculated for Albuera, for several volleys at 100 yards the British achieved 5 % ratio of casualties.
• During one of the battles of the Revolutionary Wars, General Duhesme found his battalion firing at Austrian battalion at 100 paces. It was a lengthy firefight and Duhesme expected heavy casualties, he was however very surprised, there were only 3-4 men hit.
• In 1813 at Gohrde, 66 French infantrymen fired at 60-80 paces at Germans hiting 27 Hannoverians and Bremen-Verden (40 % hits). In this case the count is only for one volley at close range.
• In 1813 at Dennewitz, a single squadron of Prussian Brandenburg Dragoons attacked a French battalion formed in square. The infantry delivered volley at 30 paces killing 23 horses and 7 men, and wounding 18 horses and 21 men. It seems poor accuracy but the horse could take several bullets before falling dead. The number of hits was probably higher than actual casualties. Eighty dragoons were untouched by the musketry. Cavalrymen were smaller targets than horses, and they ducked under fire. It made them even smaller targets.
• Mark Adkin calculated the effectiveness of muskets at Waterloo. He wrote that in the prolonged fighting for Hougoumont "it took 224 French musket shots to secure a hit. This is not such a poor performance as it seems. Most defenders (Germans and British) were behind cover of some sort for much of the time, if only a hedge or a tree. The majority were behind brick walls."
• General Gassendi of the French army calculated that only 1 shot in 3.000 resulted in casulty.
• According to Guibert only 0.2 % of all shots hit the target. All shots means all shots, not only the battalion salvos at close range repeatedly described in many memoirs. Up to 15-25 % % of all shots were misfires, many troops fired at too long distance, some of the lightly wounded went uncounted, part of ammunition nominally fired was thrown away by soldiers etc. etc. (In 1876 in the Battle of Rosebud, Crook's troops, 43 officers and 1,000 other ranks armed with modern carbines and rifles, repulsed several charges made by 800 mounted Sioux warriors. The Indians attacked with "an enthusiasm for battle seldom seen." (- Gregory Michno) The battle raged for six hours, the soldiers had expended 25,000 rounds of ammunition, the Indian lost 102 killed and wounded. It gives a rate of 250 rounds/1 casualty.

The same source:

Effectiveness of muskets was low due to several factors:

• on windless day, the gunsmoke was so dense that the infantrymen could hardly distinguish friend from foe.

• ball was not tightly fitted to the bore of the musket, it came out the muzzle at no generally predictable angle

• misfires consisted of up to 20 %. According to Colonel Elting during prolonged firing the soldier had often to clear the vent with a pin carried on his pouch belt, and clean the barrel which fouled after 50 or 60 shots. Cartridges were spoiled by humidity. In wet weather men who failed to keep them dry, or to cap or wrap up their lock plates, were incapable of firing a shot.

• line infantry was not taught to aim, but simply to point their muskets in the general direction of the target. (In the heat of battle the soldier often forgot that in close range it was necessary to point the musket at the feet of a target because the kick of the musket threw the barrel up, causing the ball to arc up, to pass over the target.)

• stress. The experience showed that the niceties of regular volleys were impracticable on the battlefield. Quite often the musketry took place outside of the real killing zone. The sight of enemy continuing his advance was enough for some and they began blasting off as soon as they had loaded their muskets. It was contagious. Once individual soldiers fired their muskets (without the order from their officers) the others began firing too. Within moments the entire battalion was covered with smoke. The fast firing relieved anxiety and occupied troopers' minds and bodies. Some soldiers were so stressed that they loaded their rifles time after time but they never fired. (After one of the battle of Gettysburg the discarded 37,574 rifles were collected and sent to Washington to be inspected and reissued. Approx. 24,000 of them were still loaded, and 75 % of them had 2 to 10 rounds in the barrel. One rifle had been stuffed to the top with 23 rounds !