Box you shoot into to test a gun

Bullet trap.

There are several different kinds of bullet traps on the market. The only one I can remember the name of right now is the savage range systems one. They do sizes from simple safe clearing catch container up to full blown ranges and even have one they say is designed for gunsmiths and such.

http://www.savagerangesystems.com/bullettraps/

If you've got lots of money, or really powerful firearms, the Bullet Bunker is nice. http://thebulletbunker.com/

most of us just call them a "sandbox". i have one made from a 20 gallon steel drum that is filled with gravel and has a standard ring clamp lid that has the center cut out and is sealed by a piece of 1/2" rubber sheet. about 18" of gravel is enuf to stop anything i have thrown at it yet.

A 5gal bucket of sand will stop just about any normal sporting or SD caliber.

rc

if you want to inspect the rifling grooves cut into the projectile a simple sand trap will deform the bullet, but we get by with what we got, often enough surface is visible..
The traps that slow down the projectile in stages are elaborate and expensive..
you can tap the bullet through the bore with a wood dowel but sometimes it can turn into a nightmare if (when) it gets stuck :banghead:

A garbage can filled with water will stop most handgun bullets, and allow the rifling to be tested.