Perhaps I can bridge the gaps in communication that I see here.
- I think all parties agree that accuracy of an individual musket was typically low.
- I think we also all agree that fire by formed bodies was typically at a range of +/- 100m, often less.
- Tests also showed that a musket ball travelled much further than 100m, but that it was all but impossible to aim. As Digby points out, smoothbore projectiles have inconsistent spin due to the lack of rifling. The balls were also loose within the barrel compared to a modern weapon so the trajectory on leaving the muzzle would differ from shot to shot. I would liken each individual shot to a single pellet from a shotgun - you can be confident of the cone within which it will travel, but not much more.
- Accounts also indicate that soldiers in the line did not generally aim, but at best, point in the correct direction at the correct elevation. This seems entirely reasonable for a weapon designed to be effective en masse, not individually. I have also seen accounts of fire being off when units were not experienced with firing on slopes, so that virtually all the fire went over the heads of the other unit.
May I suggest that Hans-Karl is not speaking of aimed fire, but rather of skirmishers simply firing into an area, and that due to distance having to do so at some elevation (hence "arcing" fire). How effective would this be? Probably not very. How many firers, at what rate of fire, with what probability of hit? Probably low, low and low. However, the morale effect of even one shot arriving could be significant. I think Hans-Karl provides a quote from an artillery officer of his men becoming quite disconcerted by such fire. I can also see more experienced units largely ignoring such fire as being inconsequential, and because nothing could be done about it.
For what it is worth, I have personal experience of delivering such fire, though with different weapons. I spent several years as a anti-armour gunner, mostly on the 106mm recoilless rifle (yes, for those know the piece, I am dating myself), including teaching courses and running ranges. The 106 was a direct fire weapon, effective to 1200m. However, we had range books for indirect fire. I know of few people who ever fired in this way. We did have a few occasions when we had excess ammunition, crews which I wanted to expose to more advanced skills, and a suitable range. The maximum indirect fire range was just short of 8400 yards. The weapon had none of the mechanisms that an artillery piece or mortar has to provide accuracy for indirect fire. Therefore all such fire was by touch. On one range we had a useful small hill about 3500m down range, visible from the firing point. It was possible to teach many of the gunners how to lob fire on that hill and hit it with some level of consistency. We did a trial at maximum range on one shoot. We levelled the weapon and aligned it by compass, and because we could not see the target from the firing point (7600m is a looong way away) it was a challenge. Quite a few of our people knew mortars as well which helped. The results indicated that against a decent target, such as a vehicle lager we could get on target, especially if we prepared the firing bed and ranged in. In practical terms it would be a harassment only as the rounds were HEAT and HEP (designed to penetrate armour), not fragmentation, so anything but a direct hit would do little damage. Still, a hit at that range would descend at a significant angle and defeat most vehicles.
For what it is worth we also experimented with elevated fire using a number of other flat trajectory weapons including the 84mm Carl Gustav, 57mm RCL and the M-72.
The long and the short is that musket fire was typically at 100m or less, but a musket ball could easily travel 600m if the weapon was elevated. I can well believe that a skirmisher could attempt to loft a ball at such a range into a target. Aimed shot? Hardly. Probability of hitting the target unit would depend on the skill of the firer (that is, how often he had been able to fire in this way), the area occupied by the target and the density of people or animals within the target area. Effect on the recipient of such a shot? Possibly a casualty. Force on the ball could be as little as the velocity imparted by gravity from the apex of the trajectory plus any remaining forward velocity. Terminal velocity for a 1 ounce musket ball, at 2cm diameter would be about 30 km/hr or nearly 9 m/s. Would that be enough to penetrate a shako? I doubt it. (Here in Canada, we routinely see hockey players hit by pucks travelling at 100km/hr, time after time through a game. The puck weighs a lot more than a musket ball and has an edge.) It would be enough to contuse exposed flesh. Note that the accounts which Hans-Karl offers speak of rarely hitting (no surprise) and in all but one of the quotes only speaks of men being wounded, not killed.
In my view you are both right. Such firing was indeed possible, and there do appear to be enough primary accounts of it to indicate that it did happen. Those same accounts indicate that the effect was more moral than physical.