OK, in that case I vote that we get someone to record a 7.62x54R Mosin-Nagant, preferably one of the short carbine versions, as they make the most impressive boom and muzzle flash of any standard-caliber bolt-action I have seen.

Seriously, I do get your point and I agree that a recording right next to the gun might pick up wanted mechanical noise, but if the microphone is a few yards away I don't think it will matter. Regardless, sample recordings of the real guns would give confirmation of the rate of fire in actual use and a point of comparison for how realistic the simulation sounds. That said, you may well be right that recording just a single shot and looping it in synch with the light would be the easiest solution.