The user will set the volume to 100 and set the volume of his speakers to hear normaly if he wants to enjoy the game. If he's playing the game just because he doesnt have anything else to do,you dont need to get his attention with gun shots
he doesnt want you to get his attention.
Btw , you can hear the sounds from far by increasing the volume , but it'll be too loud at the bullet possition. It is realistic if you do it that way,but after all this is a game
you dont want to scare the player if he is shooting.