Pink foam makes an *awesome backstop*. Just picked up a few airsoft equivalents of my real steel for practice, was going to do the towel trick, then realized I had spare foam from another project, thought I'd give it a shot (so to speak)..
Have put a couple thousand .25-.30gr into a 2x4' slab of 2" thick "Foamular 250". Have picked up a dozen off the floor afterwards.
The Tokyo Marui pistols running green gas have enough power at 12 yards that they don't bounce, just go through the paper target I've taped on, and embed in the foam up to a few BBs deep.
The WE G35 is a little slower, so get an occasional bounce towards the end of a mag.
Here's a pointer: http://www.homedepot.com/p/Owens-Corning-FOAMULAR-250-2-in-x-4-ft-x-8-ft-Tongue-and-Groove-Foam-5D/202080950
HD didn't carry it at the stores around San Jose, but I found a local distributor who was really great, sold me a couple sheets at a decent price and helped me cut it into 2x4' sections so I could fit it into my car.