I recently aquired an old Stevens model 15-A single shot rifle. I bought some Aguila Colibris and they shoot like a pellet rifle.
As crazy as it sounds, I got to wondering how I could build a bullet trap so I could shoot my rifle in the camper while I'm camped out on the job. I discovered that a newspaper, folded as it comes, fits perfectly in a large flat rate box. I taped up a box, filled it with Sunday papers that I had layin' around and taped it shut.
The box is 5 3/4" thick and 12" square. At 20 ft, it makes a perfect bullet trap. Tape a small bore target on it, stand it up at one end of the camper, plop down on the couch at the other end and shoot 'till your heart's content!