I know noobie made some silicone spacers for the vsr-10, which seemed to work well, (I could probably do the same thing pretty easily), but I don't think that the expanding foam would work well, unless it's expanded ahead of time (assuming you're talking about great stuff). I might be wrong, but FME, it doesn't expand evenly, and you'll be putting pressure on "random" parts of your inner barrel. If you use the two part expanding foam, you'll have better luck, but it still may be a problem.
Noobie is my hero
http://www.airsoftforum.com/board/No...c-t134507.html
http://www.youtube.com/user/T00BIE#p/u/9/_8eZfiecBVM