https://www.reddit.com/r/AskHistorians/comments/52xcne/a_common_claim_ive_heard_about_american/d7oucy6

See this recent discussion by /u/PapiriusCursor

It is true that soldiers in the early modern era were typically far less accurate in combat than they were on the practice field, P.B. Hughes in his book Firepower concluded that a typical musket volley scored a hit rate of only around 5% at 100 yards despite the fact that experiments in the 18th-19th centuries found volleys could hit a batallion sized wooden target 50-70% of the time at the same distance. Whether this was due to a "psychological resistance to killing" in part or at all however is not clear, there doesn't seem to be much direct evidence to support that theory, and most authors such as Hughes tend to conclude that mechanical or typical battle-stress related explanations are sufficient to account for the discrepancy.

In 1860 the US army conducted a series of experiments (Fuller, The Rifled Musket) which involved shooting various types of rifles at 10ft wide, square targets at various distances. When 10 soldiers were told to fire 5 shots each in volleys they struck all over the face of the target, sometimes performing almost as poorly as the smoothbore muskets at 100 yards. When 10 men were told to fire "as skirmishers" they were more accurate, but still generally struggled to stay on the board, never mind hitting the center. Shortly afterwards the experiment was repeated using the same weapons, but with only one soldier shooting and reloading his weapon 10 times in a row. This time the rifle bullets typically all struck an area less than 12 in across at 100 yards. This just goes to show just how much the smoke, noise, and pressure of having even 9 other soldiers shooting alongside a person can effect accuracy. Never mind on a battlefield when thousands of men are shooting at once and the enemy is a moving target which is also shooting back or charging forward with a bayonet.