I have one of these,
http://www.airforceairguns.com/talonss.html in .22, and in stock config, it has a 12" barrel with a 20" shroud, they are pretty quiet, I opted for the 24" barrel, and it is loud, 30 grn pellets being sent by 3,000 psi air, makes noise, but man is it accurate, so to answer your question, BB guns should not really make noise, as they are usually spring guns, at 200-300 fps. unless like me you have a Drozd, co2 is around 1,000 psi, and expands when released and makes noise.
Now I used to help in the international running game matches, .22 rf, rifles, and it was allowable to use what is called a bloop tube, a long tube on the end of the barrel, no insulation, just a pipe, and it was used for 2 reasons, one was weight to steady the muzzle, the other was it could be used to increase sight radius, by product, QUIETER, so me, I would go for the pvc option, cheap, lots of sizes and adapters to get real close to what ever diameter you have.
Now sound suppression devices are illegal, period, without permit, even on air guns.