Poloflece has somewhat of the idea, but changing the firerate does not affect the sounds. Here is what you want to do:
1. Go to this directory:
C:\Program Files (x86)\The Game Creators\FPS Creator\Files\gamecore\guns
2. Then choose the model pack the gun is in, then choose the gun itself
3. Open the file called, gunspec.txt, with notepad
4. Scroll down until you find fireloop and change it to something like 5000
5. It should now play the full sound. You can change the number as well to fit your liking..
6. If the ammo counter is decreasing faster than the gun is actually firing then look for firerate in the gunspec.txt and change it to something either below 4 or above 3. I would not go any farther down.