From a CSI episode: How about an empty water bottle?
Anything that captures the gases and muffles the report of a firearm will reduce the "Signature". The issue is how much.
Not that I would know ANYTHING about this highly illegal manuever, but I have heard that a 2 Liter bottle of soda when emptied and a notch cut for the sight is a slightly larger fit on a BULL BARREL of a Ruger .22 target pistol. Wrapping 100mph tape (duct tape to civilians) at the place where the two overlap MIGHT cause a significant noise reduction especially if subsonic loads are used. MIGHT. Especially for the first two or three shots. KIds....for trained professionals only....DON'T try this at home!!!!
I've read about 2 liter soda bottles making good silencers. As one poster says it can capture the gasses produced by the bullet's "explosion", thus reduce the noise