[StainlessSteelCynic]

Just to add to what the other guys have mentioned...

Another factor is Tactics, as in the tactics being used by the shooter (e.g. do they stay in one spot and repeatedly fire, do they fire single rounds or bursts, do they move locations between shots, is there more than one shooter taking alternate shots, did they wet down the ground in front of them to reduce dust, are they firing from a position where the sun is in your eyes and so on).

Not every semi-auto/single shot generates a noticeable muzzle flash either. Sometimes it's literally a "blink and you miss it" event because many modern powders are designed to reduce the amount of muzzle flash that's produced (along with the various flash suppressors/muzzle breaks on military smallarms). You may just get the after image in your eye because the flash has occurred faster than you can recognize it for what it is.