From William Muller's The Elements of the Science of War (1811)
ยง 774. On the Quickness of the Fire.
Experience has shewn that with common small arms eleven seconds are necessary to load, and with firelocks with cylindrical or equally thick ramrods, and where the powder falls by itself, through the vent from eight to nine seconds.
Consequently a body of men with common firelocks may fire 4,5 times, and with such as are provided with cylindrical ramrods five or six times per minute; but as accidents will often occur to render such expeditious firing impracticable, four or five times per minute is the quickest that can be expected from common guns.
It doesn't seem like he's using an tricks like Sharpe's "tap-loading". Was six shots in a minute actually possible for 18th century infantry? Was it even possible for most infantry to reload in 12 seconds?
Never have I come across anything that suggests that anything more than three rounds per minute is possible with even the best trained men. This comes not from the quality of men or equipment (I find it odd that he mentions cylindrical ramrods as they are common as late as the 1750s for French Charlevilles) but rather the physical limitations of flintlock muskets themselves.
In Swords Around a Throne by John Elting, his work on the organization of Napoleons Grande Armee, he spends a small amount of time on informing the reader of the weapons commonly used and how they worked. In describing a musket, he reminds us that a musket is harder to use and reload than a contemporary automatic rifle (and a note as it was written in the 80s). An automatic rifle can go from empty to fully loaded in a matter of three quick operations:
And it's ready to fire. A musket is infinitely harder to reload and thus has a larger room of error as there are as many as seventeen separate operations necessary to reload a single shot:
At the time of an empty musket.
With each step there's a chance of something messing up the speed and accuracy of which each step is done. Further each soldier is doing this in a line with other soldiers within touch distance. Shoulders and elbows can and will get in the way of each other, causing your powder to spill out or worse, making you think a misfire was a proper fire.
In the end, it's basically impossible. An ideal reload time is twenty seconds due to the amount of steps there are to reloading a musket and the safety to make sure you don't blow the musket in your face or shoot your ramrod, both of which are common occurrences.