I think the shot needs to be more loud, just take the audio from a real gun (what GBB were you using? just find audio of that real steel pistol and use that) and maybe either
a.) simulate more recoil (you're shooting with one hand so I'd think that there would be more recoil)
b.) shoot with both hands, you know like you normally would
As for the flash, it's too big and you need to add some smoke, also maybe you could add light flashing off the wall from the flash of the gun.
Just my 2 cents.