Standard Bullets and Casings

As with fingerprints, every firearm has unique characteristics and, when fired, imprints unique signatures on the bullets and casings. By analyzing these ballistics signatures, examiners are able to connect a particular firearm to criminal acts.

The NIST standard bullets and casings have been developed as reference standards for crime laboratories to help verify that the computerized optical-imaging equipment in those laboratories is operating properly. They are currently being tested across the country to develop a procedure for quality control of optical acquisitions to enable nationwide and worldwide ballistics measurement traceability and unification.

Figure 1. SRM 2460 Standard Bullet.

The standard bullets and casings were designed with the size, shape, color, and material as close as possible to real bullets (see Figure 1). The bullet signature patterns of the standard bullets come from real bullets. These bullet signatures must be highly repeatable in different axial sections on the same standard bullet, and highly reproducible in a group of standard bullets.

The numerically controlled (NC) diamond turning process was used at NIST for production of the first set of prototype standard bullets. Specially designed fixtures were used to hold the standard bullets on the diamond turning machine for manufacturing of the bullet signatures. The original bullet signatures were replicated from master bullets fired by a standardized shooting procedure at the National Laboratory Center of the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). The digitized bullet signatures were stored in a computer to control the NC diamond turning machine for the production of the standard bullets.

Figure 1 shows one of the standard bullets produced by this fabrication process. Comparison results showed high repeatability and reproducibility of bullet signatures of NIST standard bullets [1].

Figure 2. SRM 2461 Prototype Standard Casing.

The NIST standard casings project is currently in progress. Due to the three dimensional nature of a bullet casing, new techniques were needed in the replication, measurement, and correlation. The electro-forming technique has a long history for replication of surface specimens [2]. A master specimen is put into an electrolytic tank to produce a negative replica on the surface of the master. By repeating the same process on the negative replica, a positive replica is produced with the same surface topography as the master specimen [2]. In order to ensure that the SRM casings are produced with virtually the same surface topography, it is necessary to test the "decay factors" of the replication process [3]. Twenty-six prototype standard casings were replicated from the same ATF master for tests of the decay factors, two of them are shown in Figure 2. Test results showed that the decay factors are very small; this can ensure that the topography reproducibility of 256 SRM Casings replicated from the same ATF master will be high and that the casings will be nearly identical.

Figure 3. 3D Topography Correlation Program.

To measure the three dimensional markings of these casings, a confocal microscope was used to capture the topography. Figure 3 shows a screen output of a firing pin signature comparison between two prototype SRM Casings 001 and 035. The resulting 3D surface topographies are shown on the top of Figure 3. These two surface topographies are filtered by a 0.25 mm long wavelength cutoff [4] and registered along the X- and Y-directions and rotated around the Z-axis until the maximum correlation position is found, see bottom images, left and center. Once registered, the area cross correlation function maximum ACCF_max is calculated to be 98.78 %. Meanwhile, a topography difference Z_{B – A} (X, Y) is calculated at this position and shown in the bottom right in Figure 3. The signature difference between the two 3D topographies is calculated to be D_s = 2.30 %. The first set of 102 standard casings have been measured and correlated. Another 71 casings are in the queue to be measured and correlated, and another 83 are being manufactured.

References

[1] Ma, Li, Song, Jun-Feng (John), Whitenton, Eric P., Vorburger, Theodore V., Zhou, J., Zheng, A., “NIST Bullet Signature Measurement System for RM (Reference Material) 8240 Standard Bullets,” Journal of Forensic Sciences 49 (4), (July 2004).

[2] Song, J., Vorburger, T. and Rubert, P., “Comparison between Precision Roughness Master Specimens and Their Electroformed Replicas,” Precision Engineering 14 (2), pp. 84-90 (1992).

[3] Song, Jun-Feng (John), Rubert, P., Zheng, Xiaoyu A. (Alan), Vorburger, Theodore V., “Topography Measurements for Determining the Decay Factors in Surface Replication,” Journal of Measurement Science and Technology 19 (8), pp. 1-4, London, United Kingdom (2008). DOI :10.1088/0957-0233/19/8/084005.

[4] ASME B46.1-2002, Surface Texture–Roughness, Waviness and Lay, ASME, New York, New York (2003).