Why not take the shot somewhere very dark with a 2 second exposure with a pre flash on the background and rifle or do two shots one in the dark then one for the background / foreground and layer them in PS? might work, then again it might not...
You would get the muzzle flash but also the recoil of the gun.
I tried it trying to shoot a .45 pistol......didn't get one with a 40D at 6fps at 1/250. It is not that easy I can tell you, the muzzle flash is only around for a millisecond and you need to be at EXACTLY the moment to catch. Good luck!!
Graham - I don't think you are right saying that at 8fps you are bound to capture a bullet. Even the slowest bullets have a muzzle exit velocity of 1000 feet per second...!! You would be extremely lucky to get the bullet in frame, because even at 1000th sec shutter speed, the bullet would have traveled a distance of 1 foot.Or at 8 fps second you are bound to catch something if you just hold the button down and they do the same