CLANK!
Durable but not very quiet. 4 inches of duct seal putty in the back of a deep wooden box seems to stop even a 20 FPE .20 cal pretty easily. If you shoot repeatedly at the same bull, after a few hundred shots, you are actually shooting into a mega-wad of lead embedded in the putty, but it is still quiet and probably even better at stopping the pellets. When the accumulated lead gets to be a nugget about an inch or so in size, I dig it out with a screwdirver (careful - there are sharp metal edges on the nuggets), push the putty back in the hole (again with the screwdriver), and start over. The nuggets can be recycled if you choose. I have accumulated about half a dozen so far. Size of nugget may vary with accuracy of the shooter. If you look carefully, you will see that I have started shooting 5-Bull targets, too. The quarter round facing was added to protect the original wood of the target. When I bang it up badly enough, I will replace the quarter round, too.