It is a very simple silencer but it dramatically dampens the sound of the Airsoft gun.
Step 1: How to Do
It is simple. Take a cardboard and roll it to make a cylinder, Punch small holes on it. 3 holes are enough. Make sure the holes are disperse and facing different directions. Also seal the top of the cylinder with paper, than let the BB bullet to penetrate the paper. Then use some tape to harden the area around the hole.
The principle is simple, we are forcing the high pressure air to be released slowly. This is also the principle behind the real silencers.
I tried it and it works. It suppresses the sound a bit but also it makes the muzzle noise much more bass.
(note-1: this design may be much better if made up with plastic, but i didn't tried it, i don't know)
(note-2: the cylinder may be built a bit conical, thinner in the back, thicker in the front, cuz the bullet may hit the cylinder if the exit hole of the cylinder is small.)
(note-3: most of the silencers for airsoft guns are just cylinders without holes, so it just tunnels the air and makes the sounds more bass, but doesn't dampens the sound at all. This one uses the principle for real silencer so it actually works even it is a very simple design)
(note-4: diameter of the holes should be 3 or 4 mm)