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As with fingerprints, every firearm has unique characteristics and, when fired, imprints unique signatures on the bullets and casings. By analyzing these ballistic signatures, examiners are able to connect a particular firearm to criminal firearm investigations. The NIST standard bullets, Standard Reference Material (SRM) 2460, and standard cartridge cases (SRM 2461) have been developed as reference standards for crime laboratories to help verify that the computerized optical-imaging equipment in those laboratories is operating properly and to facilitate ballistics measurement traceability and laboratory accreditation. They have been tested in volunteer crime laboratories across the country for the purpose of developing quality control procedures for optical acquisitions in the National Integrated Ballistic Information Network directed by the Bureau of Alcohol Tobacco, Firearms, and Explosives (ATF) .
The standard bullets were designed to have 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 actual fired 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 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 ATF and at the Federal Bureau of Investigation Crime Laboratory. 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 . Surface topography profiles of the land engraved areas of the SRM 2460 Bullet may be found in the 2D Virtual SRM Database section of the web-based Surface Metrology Algorithm Testing System (SMATS).
Standard Cartridge Case
Standard Reference Material (SRM) 2461 is a physical standard that provides markings of a fired cartridge case. Each unit of SRM 2461 consists of a circular electroformed nickel plate about 1 mm thick, replicated from the head of a fired master cartridge case and cemented to a brass cylinder holder (see Figure 2) so that the assembly resembles an actual fired cartridge case. The electroformed plate contains a surface topography signature of a breech face impression, a firing pin impression, and an ejector mark. In order to protect the outer surface of the SRM cartridge case, the diameter of the brass cylinder (about 12.7 mm) is made larger than the diameter of the cartridge case (about 9 mm).
Certified Areal Cross Correlation Function Maximum ACCFmax and Signature Difference Ds: Two properties of the surface topography are used to characterize the similarity of the cartridge case surfaces: the areal cross correlation function maximum ACCFmax and the signature difference Ds . The certified values are obtained from statistical correlations between the surface topography of breech face, firing pin and ejector mark regions of the SRM 2461 cartridge cases and those of a reference standard, comprised of the surface topography of breech face impression, firing pin impression and ejector mark captured from three reference SRM cartridge cases with Serial Numbers 155, 153, and 260, respectively. When two correlated cartridge case signatures are exactly the same (point by point), Ds is equal to 0 and ACCFmax must be equal to 100 %.
Topography Images: This website contains topography images obtained from the breech face impression of unit 155, the firing pin impression of unit 153, and the ejector mark of unit 260 with which the user can correlate topography images obtained from one of the distributed units of SRM 2461. Links to the topography image data are given below:
- SRM 2461, Unit 155, Breech Face Impression, Raw Topography Image
- SRM 2461, Unit 153, Firing Pin Impression, Raw Topography Image
- SRM 2461, Unit 260, Ejector Mark, Raw Topography Image
- SRM 2461, Unit 155, Breech Face Impression, Filtered Topography Image
- SRM 2461, Unit 153, Firing Pin Impression, Filtered Topography Image
- SRM 2461, Unit 260, Ejector Mark, Filtered Topography Image
For all topography images, the data are in ASCII tab delimited format. The first row represents the x-coordinates and the first column represents the y-coordinates. All other numbers represent the z-height values for each corresponding x- and y-location. All units are in micrometers. For the raw topography images, dropouts (bad data points) are set equal to the lowest z-value in the entire matrix; for the filtered topography images, the dropouts are set equal to zero. Dropouts should be ignored during any type of analysis. The topography images for the breech face and ejector mark were manually trimmed to eliminate irregular edges and headstamp marks from the data to be analyzed.
The filtered topography images result from bandpass filtering to minimize form and waviness and to emphasize the fine roughness features of these measured topographies. The conditions of filtering and preprocessing of the raw topography images are described below in the text associated with Figures 3, 4, and 5. The figures also show the raw and filtered topography images of the breech face impression of Unit 155, the firing pin impression of Unit 153, and the ejector mark of Unit 260 measured at NIST. Table 1 below shows the results for the parameters ACCFmax and Ds as obtained from NIST measurements.
Electroformed Replicas from a Master Cartridge Case Fired at ATF: The master cartridge case was fired at the National Laboratory Center of the ATF . The signature reproducibility of SRM cartridge cases depends on the master cartridge case and on the electroforming process. In order to ensure that the SRM cartridge cases produced from the same master have virtually the same surface topography signatures, it was necessary to test for differences in the replica surfaces . The results showed that the electroforming process was stable and was capable of producing a large number of identical surfaces.
Measurements and Analysis
A total of 175 SRM 2461 standard cartridge cases, with serial numbers S/N 104 to S/N 278, were measured for their breech face, firing pin and ejector mark topography images, and the topography images were correlated with those of the three reference standards. The multiple correlation results were then statistically analyzed. The NIST topography measurements were performed with a confocal microscope and topography images were obtained for the three areas. On the breech face and firing pin areas, the cylindrical axis of the cartridge case was parallel to the optical axis of the microscope. However, for the topography measurement of the ejector marks, the SRM cartridge case was tilted approximately 20 ° so that the principal surface of the ejector mark was aligned perpendicular to the microscope optical axis as well as possible.
The values of ACCFmax and Ds for the breech face, firing pin, and ejector mark signatures were reported with a 95 % confidence level (α = 95 %) [4,5]. For all three regions of the 137 SRM cartridge cases being distributed, the lower limit for ACCFmax and upper limit for Ds, each with a 95 % confidence level (α = 95 %) are reported in Table 1. A NIST certified value is a value for which NIST has the highest confidence in its accuracy in that all known or suspected sources of bias have been investigated or accounted for by NIST.
The reference standards for topography correlations of breech face, firing pin and ejector mark of the SRM cartridge cases were captured from three different reference SRM cartridge cases. The breech face signature standard was captured from SRM 2461-155; the firing pin signature standard was captured from SRM 2461-153; and the ejector mark signature standard was captured from SRM 2461-260. In order to evaluate the uniformity and reproducibility of the cartridge case signatures between the distributed SRM 2461 cartridge cases and the reference standards, two parameters are used to quantify the similarity of a pair of cartridge case topography signatures . One of these is the ACCFmax, the maximum value of the areal cross correlation function defined as 
where the sets of points, Amn and Bmn, exclude any data dropouts and outliers. The ACCFmax is the maximum value of the areal cross correlation function ACCF , which occurs when the image B of the SRM cartridge case and the image A of the reference standard are registered at their maximum correlation position.
Before performing the correlation, the topography data are processed by
- locating and interpolating through data dropouts and outliers,
- applying a least squares fit to a second order polynomial to reduce form error,
- applying a long-wavelength Gaussian filter to further reduce any remaining form and waviness error and applying a short wavelength Gaussian filter to reduce noise,
- removing the data dropouts and outliers from further analysis,
- registering the two topography images by allowing x-y translation and z-rotation of the compared image until the position of maximum correlation is achieved.
At the maximum correlation position, a difference image Zmn is calculated, which equals the difference between the compared and reference images, Bmn and Amn, respectively:
The second parameter, the signature difference, Ds , is defined as a ratio of the areal mean-square roughness Sq2  of the signature difference image Z to the areal mean-square roughness of the reference image A:
When the compared image B is exactly the same as the reference image A (point by point), Ds must be equal to 0 and ACCFmax must be equal to 100 %.
Figure 3 shows a cross correlation between the topography images of the breech face impressions of the SRM reference cartridge case S/N 155 (top, left) and the SRM compared cartridge case S/N 174 (top, right). The cutoff filters were moving average summations representing close approximations to Gaussian weighting functions. The long wavelength nesting index  (cutoff) was 0.4 mm, and the short wavelength nesting index was 2.5 μm. At the maximum correlation position, the ACCFmax is calculated to be 95.8 %. The topography difference Z is also calculated (see bottom, right), and from it, the signature difference Ds is calculated to be 8.3 %.
Figure 4 shows a cross correlation between the topography images of the firing pin impressions of the SRM reference cartridge case S/N 153 (top, left) and the SRM compared cartridge case S/N 242 (top, right). The long wavelength nesting index (cutoff) was 0.15 mm, and the short wavelength nesting index was 2.5 μm. At the maximum correlation position, the ACCFmax is calculated to be 98.9 %. The topography difference Z is also calculated (see bottom, right), and from it, the signature difference Ds is calculated to be 2.2 %.
Figure 5 shows a cross correlation between the topography images of the ejector marks of SRM reference cartridge case S/N 260 (top, left) and the SRM compared cartridge case S/N 271 (top, right). The long wavelength nesting index (cutoff) was 0.15 mm, and the short wavelength nesting index was 2.5 μm. At the maximum correlation position, the ACCFmax is calculated to be 95.2 %. The topography difference Z is also calculated (see bottom, right), and from it, the signature difference Ds is calculated to be 9.5 %.
The ACCFmax and Ds values from the measurements of the 172 SRM cartridge cases were statistically analyzed. 137 units were selected for their high surface quality in all three regions. They are reported with a confidence level of α = 95 % . With 95 % confidence for each measure, each region on each cartridge case unit has an ACCFmax value higher than the value shown in Table 1 and a Ds value lower than the value shown in Table 1. To facilitate quantitative comparisons, both the measured topography data and the processed, filtered data for all three regions on the reference standards are available via the above links.
Sources of measurement uncertainty include those from instrument noise, instrument calibration and measurement setup, topography digitization including both the quantization level and sampling interval, image distortion caused by the optical system, errors due to the stitching of three images used for ejector mark and nine images for breech face, and variations from environment and operation setup. All these uncertainty components result in Type A variations in the topography images and in Type A variations when the cartridge case topography images are correlated with the reference images. Hence, all the uncertainties in the measurement system result in variations of the ACCFmax and Ds parameters for the 137 SRM 2461 cartridge cases and are judged to be directly estimated by the observed Type A variations of those parameters.
Expiration of Certification: The certification of SRM 2461 is expected to be valid, within the measurement uncertainties specified, until 30 September 2021, provided the SRM is handled, stored, and used in accordance with the instructions given below. However, the certification is nullified for an inspected area that is damaged, contaminated, or modified. NIST reserves the right to withdraw, amend, or extend this certification at any time. NIST will monitor this SRM over the period of its certification. If substantive surface changes occur that affect the certification before the expiration of this certificate, NIST will notify the purchaser.
Storage and Handling: The SRM 2461 cartridge cases must be used and kept in a dry and clean environment at temperatures between 10 °C and 30 °C. The standard cartridge cases are expected to be robust and maintain their quality over many years. However, it is good procedure to avoid handling the surface of the head in order to avoid unnecessary scratches and finger contamination from marring it. Touching the surface of the SRM 2461 Standard Cartridge Case with bare hands may cause corrosion on the SRM 2461 surface and may damage the topography signatures, and therefore, should be avoided. Likewise, cleaning should also be avoided as much as possible because the cleaning process itself can introduce irreversible changes in the surface topography of the cartridge case. If it is clear that contamination has been unavoidably introduced on the surface to the extent that it has been visibly changed, then a mild cleaning procedure may be used. The suggested procedure is to clean only the contaminated area with a lab swab/cotton tip applicator moistened with ethyl alcohol.
 Song, J., Vorburger, T.V., Ballou, S., Thompson, R.M., Yen, J., Renegar, T.B., Zheng, A., Silver, R.M., Ols, M., "The National Ballistics Imaging Comparison (NBIC) Project," Forensic Sci. Int. 216, pp. 168-182 (2012); DOI:10.1016/j.forsciint.2011.09.016.
 Song, J., Whitenton, E., Kelley, D., Clary, R., Ma, L., Ballou, S., Ols, M., "SRM 2460/2461 Standard Bullets and Cartridge Cases Project"; J. Res. Natl. Inst. Stand. Technol. 109, pp. 533–542 (2004).
 Song, J., Rubert, P., Zheng, A., Vorburger, T., "Topography measurements for determining the decay factors in surface replication," Measurement Science and Technology 19, 084005 (2008); DOI:10.1088/0957-0233/19/8/084005.
 JCGM 100 2008, Evaluation of measurement data—Guide to the expression of uncertainty in measurement; http://www.bipm.org/en/publications/guides/gum.html.
 See the function corr2( ) in MATLAB, The MathWorks, Inc., http://www.mathworks.com.
 Vorburger, T.V., Yen, J., Bachrach, B., Renegar, T.B., Filliben, J.J., Ma, L., Rhee, H.-G., Zheng, A., Song, J., Riley, M., Foreman, C.D., Ballou, S.M., Surface Topography Analysis for a Feasibility Assessment of a National Ballistics Imaging Database, NISTIR 7362 (National Institute of Standards and Technology, Gaithersburg, MD, 2007).
 American National Standard ASME B46.1 (2009), Surface Texture (Surface Roughness, Waviness, and Lay), (Amer. Soc. Mech. Engrs., New York, 2010).
 ISO 25178-2:2012 Geometrical product specifications (GPS)—Surface texture: Areal—Part 2: Terms, definitions and surface texture parameters, (International Organization for Standardization, Geneva, 2012).
Note: Users of this SRM should ensure that the certificate in their possession is current. This can be accomplished by contacting the NIST SRM Program at: 301-975-6776 (Telephone); 301-926-4751 (Facsimile); email@example.com (Email); or http://www.nist.gov/srm/ (Internet).