There's a neat little secret to getting those shots:
Place your finger on the shutter button with
j-u-s-t--e-n-o-u-g-h pressure to hold it at the threshold of releasing but DON'T try to time the shot. Just wait till the Gun Captain lowers the slow match to the touch hole. Until then, concentrate on focus and framing, etc.
When the gun goes off, the shock wave from the explosion will make you flinch. Your reflexes will cause you to set off the shutter at the right moment.
If you can put the camera on a tripod or at least a steady surface like a rock or a tree stump, it will help immensely. Those two pictures I took were shot with the camera resting on the top of a wooden fence post.
If you try to time the shot you will not get very many good pictures, if you get any at all because the very same reflex action that you "should" be using to trip the shutter for you will cause you to flinch and spoil the shot.
It is virtually impossible for any but the most "hard boiled" cannon crew to keep from flinching.
And that's only because they're probably deaf as a stump!
