First question, I suppose, is what the hell are guns doing in a sword and sorcery fantasy? In short – I’m writing this story and I get to choose. When most of us think about guns and gunpowder images of white smokeless powder come into our heads. This is the stuff that you’ll find inside of modern cartridges, let’s say a .22 round, for example. My story doesn’t involve this sort of thing, otherwise it would be a lot easier. For my purposes, I’m more interested in black powder, the sort of stuff you’d find on an old wooden tall ship, or in a revolutionary war musket.
This proved to be one of the more difficult things to easily dredge up. Largely because there are thousands (millions? possibly approaching trillions?) of blogs and forums on the interwebs about guns, and you have to wade through a lot of unhelpful junk in order to find anything useful. Even then, since guns and gunpowder are so dangerous, there’s not a lot of easy to find general information out there. Most folks offering advice have an interest in making sure you don’t accidentally blow yourself up. Let’s say we ask the question: What is the volume of a pound of black powder? The answer you’re likely to find time after time is: ‘it depends’. What about: How much powder is required to fire a cannon? Again, this can be a ‘it depends’ situation. As a hapless writer who really just needs to know what is reasonable and plausible, none of this ‘it depends’ crap is helpful. In the end, I did manage to locate enough detail to sketch out an island of ‘reasonable’ for my story, here are my notes on the topic:
Black gunpowder consists of roughly seventy-five parts of saltpeter (Potassium Nitrate KNO3) to twelve and one-half parts of sulfur, and twelve and one have parts of charcoal. Once upon a time, Saltpeter was extracted mostly from animal waste through various methods OR could be mined. Either way producing it was NOT cheap (http://mysite.du.edu/~jcalvert/tech/cannon.htm)
Powder kegs are considerably smaller than kegs to transport anything else. Most traditional oak barrels run anywhere between 160-300L (about 40 to about 80 Gallons), which take up a space of something like 28” in diameter and 35” tall. One source (I’d cite it here, but I didn’t write it down. I think it was associated with the US Civil War, or possibly the British navy. In any case, I couldn’t find it when I went looking.) describes a common powder keg as containing 25lbs of powder, which translates to a container roughly 10” in diameter by 14” tall. This makes sense, because it’s a very transportable size.
The consumption of powder for a cannon can be assumed to be 1/3rd the weight of the ball (Can’t locate the source for this again), for example, a 32 pounder would consume roughly 11lbs of powder per round (http://www.cannonsuperstore.com/cannoninfo.htm). If you scale this down for a musket (and I suspect it scales reasonably well), the amount of powder consumed per shot is negligible compared to the volume of a full powder keg, something on the order of 6-8 grams (http://www.chuckhawks.com/blackpowder_volumetric.htm). You could easily get 1500-1800 rounds out of a powder keg.
And finally, (only because it’s in my notes, and was actually important for my story) rust rate of cannons:
Basically, if you have a cannon sitting in open air next to the ocean for five hundred years, after a bit of cleaning, you could still probably fire it.
Anyhow, all of this looking around got me enough information to write the bits I needed to, and I think what I have seems plausible. Perhaps not perfectly accurate, but good enough.