I've been to a couple of English Civil War re-enactments in the last two months. Two things surprised me in the first. One was that the muskets weren't as loud as I expected, and the other was the very clear delay between the primer and the main charge igniting. (You can see this in this photo by my friend. Edit: actually, technically this one was an Elizabethan militia, hence the crested helmet.) At the second re-enactment I noticed that the shots were considerably louder, and this delay wasn't noticeable.
The difference - the first was on a damp day, the second on a dry, sunny one. I don't think it was actually raining when they were firing, but it had been drizzling, and it was humid (I think I noticed them having some trouble pouring the powder out of the flasks).
That got me thinking. How much does the state of the powder affect the muzzle velocity of the ball, and thus its range and penetration? Are there any studies on this?
/u/DonaldFDraper and others ( including me) answered a question here about how armies would be using muzzle-loading guns, even in the rain.
I'm not sure but what this rather technical question might have been better posted at r/Science or r/Firearms, but I will try to provide a useful answer.
Black powder is a mix of sulfur, charcoal and potassium nitrate. It creates a lot of gas when the sulfur and potassium nitrate get heated enough to react with the charcoal ( technically, when the sulfur is heated enough to react directly, and the potassium nitrate is heated enough to release oxygen to react with the charcoal) which creates mostly potassium carbonate ( solid) , potassium sulfate ( solid) carbon dioxide ( gas) and nitrogen ( gas). Because the phase change of water turning into steam consumes a lot of heat, if water is added to black powder considerably more heat can then be required to get the potassium nitrate and sulfur to a temperature to kick off the reaction. In other words, it becomes harder to set off, and is slower to burn. For matchlocks, flintlocks, this means it's harder to set off the gun, and dampness can result in a hang-fire ( cha....boom) or no fire at all. But if the powder charge in the barrel is dry enough to actually be set off by the flash of powder in the pan, there's not much variation in muzzle velocity with damp or dry weather, as the reaction is completed and propellant gas generated almost immediately if the charge is closely packed. As the reenactors you saw were shooting blanks, it might have been different- the powder would not have been packed, would be somewhat loose in the barrel without probably even a wad to press it together. And of course that's also why the guns seemed rather quiet. Though not as loud as modern high-powered rifles, if they actually had been loaded with ball they would have been much louder.
EDIT W.W. Greener, in his The Gun and Its Development (1910) states that the fouling left behind in the barrel had enough water in it to affect the next load, as it would be rammed down into the next charge with the ball and wad. As Greener was a gunmaker considerably interested in promoting the advance of technology- especially his own-and thus the abandoning of old methods, I am not sure it's a reliable statement.
To add on a bit to what /u/bodark43 wrote. Wet weather was identified pretty early on as a potential weakness of firearms, although 16th century mercenary Humphrey Barwick argued that a good soldier should know how to keep his powder and match dry in the rain, and that if it really was "so wet that everything is wet" then it would be bad weather for archers and infantry to attack in as well.
The amount of power packed in gunpowder could sometimes vary quite a bit depending on the manufacture, quality, and grain size. But assuming the powder was just damp enough that it still burned but didn't burn completely then close range penetration might be affected pretty heavily but range, probably not so much. This is due to the fact that air resistance on a round bullet is not linear, and affects faster bullets much more heavily than slower bullets.
To use the matchlock tested during the 1988 experiments in Graz, Austria as an example, at 100 meters the velocity of the bullet had already dropped from 449 m/s to 264 m/s. In other words if the muzzle velocity of the weapon were reduced by a whole 41% then the maximum range of the weapon would only be reduced from 957 m to 857 m.
