There are some examples in the American War of Independence (or maybe War of 1812), where militias would lie down to reload and break the discipline of the overall force.
Another practical aspect is sound / smoke. Letting the force fire at will would probably mean the officers could no longer really be heard. The problem certainly didn't exist with bows.
Another practical aspect is sound / smoke. Letting the force fire at will would probably mean the officers could no longer really be heard. The problem certainly didn't exist with bows.