Fire once, wait for the sound of the casing drop to complete, fire again. You will notice that the sound has changed to another variant - this is the correct expected behavior.
Shot single mode several times, do not wait for the sound of the casing drop to end, press the trigger again and again. You will notice that the game plays the same sound of the casing drop for each shot. This will continue until we give a enough time between shots.
Fire in bursts mode, you can clearly hear that from the entire collection of sounds only one variant is played the one that was selected by the game during the first shot of your burst.
CfgWeapons → WEAPON CLASS → soundBullet[]={};