We have almost since the board was started had a catalog of cannon accidents. There is a link in the reference stickies to these accidents...within the post is additional link to other accidents.
Cannon Accidents!!! Safety Warning!!!Here is the interior link.
List of past cannon accidentsThe link Dave SB posted is a newer version of that second link and I will add that to the main post...thanks Dave for posting.
Reference the thunder mug. There is one simple thing that can be done to focus the muzzle blast away from the shooter that I learned from my days of building muzzle brakes for a world famous muzzle brake company. Make an angled "crown" on the muzzle- simply touch of a tool in the lathe at the standard 30 degrees on the muzzle and make a flat no more the 1/4 inch and as little as 1/8 inch. Then when the gasses initially escape at the muzzle, the gases will be directed at angle up instead of straight 90 degrees when they exit the muzzle. The German documents that the company had where I worked studied the gas redirection at length for use in heavy mobile artillery pieces to reduce the effect of recoil on the vehicle. They also study gas redirection to reduce dust signature, and damage to adjacent equipment and men. This angle was was all that was needed to deal with that problem.
As to fuse, to long a fuse can be just as dangerous as to short a fuse. Definitely use fuse to light the mug. Know your fuse burn rate. Use enough fuse to allow you to get well away from the mug, say 15 seconds. Gone are my days when I could do the 100 yard dash in 9.5 seconds---okay okay, 10.5 maybe...But a I'll bet I can get 100 yards away in 15 seconds.
As to powder out the vent. If you are using the right size of powder, this shouldn't be an issue...but human nature such as it is....when you cut your fuse, like Tracy says consider the possibility the powder might run back out the vent. This seems the perfect place for a fused quill. Make quill with a 15 second fuse attached. If not for your fuse, time it from the vent to ignition point. You should also know how long it take to burn from vent to powder.
As Tracy stated, watch which direction the vent is pointing.. point it down range.
Also watch the wind direction you do not want to discharge this thing up wind and have burning debris raining done on the the spectators.
We can minimize the the chance of accidents by following safe procedures and to do that we just have to think first.