Being old fashioned, I use a 33mm SLR. Use a slower film as 100, and shoot near evening when the light starts to dim. You will have a narrow window of time and may get some blur. The slower shutter speed helps to get that perfict shot. I use a tripod, low to the ground. Don't extend any legs and count down to when the gun goes off, then shoot, shoot, shoot. Shoot lots of film, It's the best way to "catch the moment".
In the gallery of my website, under cannons firing, is a picture of a mortar with the soda can just leaving the the tube. That shot took lots of film.. May get one good picture a roll. I think that is the mortar Radio2 now ownes.
Good luck and let us know how you do.
Rick