in Omaha when tom Hanks is up at the bunker and when he is waiting for the Germans to come out of the bunker, when they come out he fires four shots but only two are seen and heard. if you look closely you can see the bolt move four times and the flash only twice.
Rifles can fire what are called "tracer rounds", these are rounds in the magazine which are made to light up when fired, and they are put at intervals throughout the magazine. they can come in different intervals. thats wy there isnt a muzzle flash every time. however, despite this, i believe that this still could be a slip up in the sound editing process.