Preparation
The most commonly used formula is a "10% ballistic gelatin", which is prepared by dissolving 1 part 250 bloom gelatin into 9 parts of warm water (by mass), mixing the water while pouring in the powdered gelatin. It is chilled to 4 °C (39 °F). The older NATO formula specifies a 20% solution, chilled to 10 °C (50 °F), but that solution costs more to prepare as it uses twice the amount of gelatin. In either case, a 1988 research paper by Martin Fackler recommends that the water should not be heated above 40 °C (104 °F), as this can cause a significant change in the ballistic performance.
To ensure accurate results, immediately prior to use, the gelatin block is "calibrated" by firing into it a standard .177 caliber (4.5 mm) steel BB, from an air gun over a gun chronograph into the gelatin, and the depth of penetration measured. While the exact calibration methods vary slightly, the calibration method used by the INS National Firearms Unit is fairly typical. It requires a velocity of 183 ± 3 m/s (600 ± 10 f/s), and a BB penetration between 8.3 and 9.5 cm (3.25 to 3.75 inches). This velocity is two to three times faster than that of a BB propelled by a typical spring-air BB gun, and so where a typical BB gun might not penetrate the elastic skin of an animal, the higher velocity calibration shot will penetrate the gelatin more than might be expected.
In his book Bullet Penetration, ballistics expert Duncan MacPherson describes a method that can be used to compensate for ballistic gelatin that gives a BB penetration that is off by several centimeters (up to two inches) in either direction. MacPherson's Figure 5-2, Velocity Variation Correction to Measured BB Penetration Depth, can be used to make corrections to BB penetration depth when measured BB velocity is within ±10 m/s of 180 m/s. This method can also be used to compensate for error within the allowed tolerance, and normalize results of different tests, as it is standard practice to record the exact depth of the calibration BB's penetration.
Ballistic gels made from natural gelatin are typically yellow-brown in color, and are not re-usable. Newer synthetic substitutes are engineered to closely simulate the ballistic properties of natural gelatin, while being colorless and crystal clear. Some synthetic gels are also re-usable, since they can be melted and reformed multiple times without affecting the ballistic properties of the gels.
Read more about this topic: Ballistic Gelatin
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