Asomodai

it's to distinguish them from real firearm suppressors, most airsoft suppressors won't survive being put on the end of a real gun.

the majority of airsoft ones i've seen tend to have some kind of foam filling, usually foam donuts with a spring going up the middle. you can do some creative things with them but normally you'd expect the action noise to be a lot worse.

that said, on a boltgun, gas NBB or hpa system they can be very effective when the action doesn't make much noise they really do make a difference.

@Albiscuit. Its more for suppressing the noise out of the end of the barrel then the gearbox noise. Hence only for the bolty! 

Having said that I need to also soften the blow of the piston head on cylinder as well a bit. 

@Adolf Hamster Are there any particularly good suppressors out of box that are as good as some of the expensive stuff like our friend Novs?

can't really say for brands, most of mine have come either indirectly (ie with the gun) or just cheap amazon special stuff.

you can mess around with the internals on them, stuff them with whatever you think will lessen the noise, of course the science (or possibly black art) of sound deadening isn't something i'm that good at.

i've not really done much scientific testing, but tbh i'd suspect the best thing to have is a lot of volume in which to catch the sound, but even a stubby suppressor can do an awful lot to drop the noise of the bb leaving the barrel.