Penetration testing in earlier years used to involve using thick brown paper sheets or strawboards. These were stacked together horizontally and shot at, and the count of the number of sheets or boards penetrated were tallied up and compared to each other. The following image shows one of these test racks
In modern times, the medium of choice is a material called ballistic gelatin. The reason for using ballistic gelatin is because it has about the same density of human or animal tissues. Ballistic gelatin is also preferable to actual muscle tissues, since its properties can be more carefully controlled to produce a consistent medium for doing multiple comparative tests. It must be noted that ballistic gel doesn't completely simulate actual body structure, since it doesn't have any skin or bones, which are much tougher and harder than flesh tissue.
The standard formula used for testing is called "10% ballistic gel". It consists of mixing 1 part by mass of powdered ballistic gel formula with 9 parts of water at a temperature of 54.5 C (130 F). The mixture is poured into standard molds (per the INS National Firearms Unit test, the standard mold size is 6" x 6" x 16" for handguns. Other standards bureaus may have different standard test sizes) and the mixture is chilled to 4 C (39 F) and allowed to set.
Before conducting the actual tests, the gel block is initially calibrated by firing a standard 4.5 mm. steel ball bearing from an air gun, over a chronograph and into the gel block. The air gun should shoot the ball bearing at a velocity of 183 ± 3 meters/sec. (600 ± 10 feet/sec.), which can be verified by the chronograph. If the gel block was prepared correctly, the penetration of the ball bearing should be between 8.3 to 9.5 cm. (3.25 - 3.75 inches). If this is the case, then the gel block may be used for standardized testing.
In most police agencies, testing teams usually test for penetration in other materials as well: gelatin, heavy clothing, steel, particle board, plywood, automobile glass etc.
For homebrew testers who don't have access to ballistic gel, people generally use wet newspaper blocks or wet phone books or a line of 1-gallon jugs filled with water.