The problem is also that when you're talking about a "musket" as a generic type of weapon, you're actually trying to talk about a Roman Gladius, a high-medieval 1½-hand sword and a renaissance rapier at the same time.
There is a huge difference in whether you're talking a late 15th century hand-cannon, a 16th century musket, a 18th century musket from napoleonic areas or a rifled forward-loader from the mid 19th century (the Danish, French and Russians still used musket in the 1860s - it wasn't until the German-French war in the 1970s that backloading rifled firearms were getting common place, and these were quickly superseded by cartridge-firearms such as revolvers).
I have no evidence or first-hand knowledge of how they performed, as Deleriad is pointing out is missing from this argument. But we also need to establish for what period we're trying to determine anything.
As far as I know then shots from early hand-cannons were pretty large and deadly, but very, very inaccurate and dangerous to use. I once heard at a Medieval faire that in the 1500s, it was commonplace for knights in Gothic Armour to carry a loaded pistol at their side. When they met other cavalry, they would shot the first one they met and then fight on with very ineffective swords. If this is true, then a handheld pistol could punch through Gothic armour (nearly impenetrable by non-armour piercing weapons) with ease... albeit with a short range.
The same is probably true for early muskets, since if this wasn't true, why would armour have lost it's usefulness?
Later on, when people stopped using armour, I believe bullet sizes were reduced and thus would have done less damage. Now people simply didn't use armour as it was too cumbersome to use it, if fighting by the day's military standards. Muskets were still very, very inaccurate - though you could produce rifled firearms, but these were too expensive to give to common troops - so they were mostly used by hunters, skirmishers etc.
Also, according to a book I have following the Siege of Dybbøl in Denmark in 1864, most casualties were either from shelling (which was very new! a precursor to trench warfare), or from diseases from the wounds. It was most common to get wounded and then die from the disease that wrecked you afterwards, than to die from the wound itself.
Soo... depends on period, but I think firearm damage can better be described using new CMs than opping damage.
- Dan