Hello all! I've been watching this forum a bit over the past couple of days, and would like to add some alternative ideas that could help you, Master Beowulf. Having worked in the film industry for 15 years, and taken part in many productions and events, I've run across alternatives to black powder. Mostly useful for cannons (but could be adapted for hand-guns as well), calcium carbonate, when mixed with water forms acetylene gas. There are a number of groups out here in BC who use it at parades and the like. It won't produce quite the smokey blast that gunpowder can, but a most satisfying pop. Once you've sorted out the actual gun regulations, you may want to look into this as a less restricted alternative to black powder (you can still purchase little toy cannons on the internet with calcium carbonate to fire them with). To see the kind of nice bang and fireball you can get with acetylene, you can view our exploding gingerbread house from last year.
We did the video for a Discovery Channel 15th anniversary submission, and did it in Mythbusters style. If you don't want to watch the whole video, go to about 5 minutes in where I fill the gingerbread house with acetylene, and watch from there. We used an acetylene tank, but calcium carbonate works just as well. A model rocket igniter triggers our blast, but some groups use a match or wick to the touch-hole, or a modified spark plug arrangement. You could also look into some local farm or orchard suppliers, they sometimes sell bird deterrant cannons, which fire a loud boom every few minutes to scare birds away from the crops. They may be propane powered (more readily available and somewhat safer), but could probably be modified to suit a pirate cannon.
REMEMBER! This is very explosive stuff (like black powder isn't...), so use carefully, and always outdoors. Enjoy!