I see no reason why not to experiment with sugar for firing thins out of tubes, however I would not fire live projectiles (sarshells ect). I think you will find that it is not used for reasons other than that it has not been tried.
For making fireworks I suggest you work on your Black powder, it is obviously not up to scratch if R-Candy burns faster! I suggest you tale a deep breath and plunge into the "making Black powder" thread on this forum. I think you will find that you do not have a good ball mill, or a ball mill at all, or that you are using inferior charcoal. This seems to be the cause of 90% of BP related problems.
One more thing, you should be very careful using PVC for mortars. It is not common practice for safety conscious pyros to use PVC for anything larger than a star gun, and even then one must be careful not to over lift. Certainly one should not be firing shells from a PVC tube.
Hey mate, yeah I've definitely had a good look at the BP thread. I don't have willow, but I use red wood for charcoal, it's not optimum but is still quite good. My sulphur, charcoal and KNO3 are super fine and weighed out on digital scales to 0.5 of a gram in accuracy. Obviously minus the fact I don't have a press and I don't have a mill, preparing the BP in the CIA method followed by ricing the powder is the best I can produce. The BP launches quite high, I only wish to see if I can launch it higher...
In regards to launching from PVC mortar, your right, for sure. If it were to explode in the pipe for what ever reason then it would send small sharp pieces of PVC flying. I use protective eye ware and stand at a safe distance in a large open area. I've used pressure pipe for (air guns and gas cannons) before and I believe although no where near as safe as steel mortars, they work quite well, they have had a 100% success rate so far, for all launches I even tested the pipe a few times (safely) buy pushing a shell in way tighter than I would ever normally pack it and each time it blew the shell out.