A friend and I came up with a pellet catcher that might work with BBs. We took a metal electrical box and stuffed it full of plumber's clay or putty roughly 2-3 inches worth. Then used magnets to hang targets. Works very well though cleaning is a bit of a pain especially with pellets that penetrate deep into the clay. Everything was found at a Home Depot all together for $60, had it since September and haven't changed out the putty yet.
I also did a cheap solution for at home. Took a box, stuffed the shooting end with 4 inches of scrap crumpled paper, behind that would be a phone book or a couple Ikea catalogues, then a 1.5 inch thick metal cookie tray (or 3 inches of the cheap aluminum stuff). Tapped it all up and catches my .45 metal BBs from my airguns like nothing, figuring 6mm BBs shouldn't be much different. Still keep shooting glasses on just incase of ricochettes but haven't had a problem so far.
"Hey I'm the first one to say its a great country but its a straaaaange culture. This is a place where gun store owners are given a list of stolen credit cards but not a list of CRIMINALS and MANIACS. And now they're thinking of banning toy guns...AND THEY'RE GONNA KEEP THE F**KING REAL ONES!"
-George Carlin 1937 - 2008 (RIP buddy)