The majority of the noise will be escaping from the ejection port. Basically that is a direct pathway to all the moving parts and the gas escaping from there makes a lot of noise on it's own. In short unless you have a way to cover that opening (perfect way would be foam and a box with holes drilled into it covering the whole port) to basically suppress the noise coming out of it. How you would want to go about doing this is up to you. The main ways I can think of would either look really tacky or you would have to permanently modify the gun.