Honestly, I'm surprised it wasn't tried sooner...
bOing bOing is carrying a blurb about printed shotgun slugs. Apparently, they're terrible. They're made of ABS plastic, they take 30 minutes to an hour to print, and they require a lead ball to be inserted into them anyway. Hell, I never figured plastic printed ammo would be any good - the medium just isn't a good material for projectiles.
The comments, however, led me to this gem: Silly Putty as shotgun ammo. Now THAT is kind of terrifying.
<iframe width="560" height="315" src="http://www.youtube.com/embed/t1tsdfq16Ho" frameborder="0" allowfullscreen></iframe>