So there's a few sounds that come when you use a firearm. The most obvious are the sound of the small explosion of the primer, forcing the bullet and the gases outward. That is fairly loud itself. Then there's the bullet breaking the sound barrier. Suppressors are designed to dampen the first, and using "subsonic" ammo, ammo that doesn't break the speed of sound affects the second.

There's also the sound from the action of the firearm, the moving metal parts and such. That varies depending on the gun.