International Journal of Legal Medicine

, Volume 127, Issue 5, pp 951–955 | Cite as

Identifying the source of bullet wipe: a randomised blind trial

  • D. C. Kieser
  • D. J. Carr
  • L. Girvan
  • S. C. J. Leclair
  • I. Horsfall
  • J. C. Theis
  • M. V. Swain
  • J. A. Kieser
Original Article

Abstract

Objective

To assess the usefulness of scanning electron microscopy and energy dispersive x-ray spectroscopy in matching bullet wipe to the bullet.

Hypothesis

Bullet wipe can be used to match a bullet type to a crime scene.

Keywords

Bullet wipe Bullet matching Energy dispersive x-ray Forensics Scanning electron microscopy 

Introduction

Determining the weapon used in a crime scene has become a cornerstone of forensic investigations. Multiple techniques to determine the offending weapon have been described, including the analysis of gunshot residue, cartridge cases, bullet fragments and rifling patterns [1, 2, 3, 4, 5]. Each technique has its advantages and disadvantages. For example, gunshot residue, which is principally the burned and unburned primer and propellant particles which are deposited as a cloud of material expelled from the barrel after firing, can be used to determine the weapon used, but is limited to close range or contact injuries [4, 5]. On the other hand, analysing the cartridge case [2] or the rifling patterns on the bullet [1, 3] allows matching the bullet to the firearm, but requires the cartridge or bullet to be found intact and in reasonable condition to allow useful analysis.

Bullet wipe is the term given to the material deposited by the bullet on its target or any structure it may contact during its flight [6]. This material typically consists of debris from within the barrel collected by the bullet as it passes down the barrel. It is composed principally of soot, but also contains lubricant and particles of primer and metal fragments from previous bullets or the barrel [7, 8]. No previous research has shown bullet wipe to contain bullet fragments from the infringing bullet itself. However, bullets themselves are composed of a myriad of elements, determined by design and the company that manufactures them [8]. If bullet debris from the infringing bullet were deposited in bullet wipe, an elemental analysis could determine the bullet’s origin.

Scanning electron microscopy with energy dispersive x-ray (SEM EDX) has been used on bullet fragments retained within the wound to determine their elemental composition, thus linking the bullet to the firearm [3, 9]. This technique has also been used once on bullet wipe, in a case where the detection of copper was used to distinguish between a .32-calibre soft lead bullet and a .38-calibre copper jacketed bullet [10]. However, the use of SEM EDX on bullet wipe to determine the bullet’s composition has not been verified.

In this study, we analysed whether we could identify bullet fragments off bullet wipe at a distance of greater than 10 m and whether bullet wipe was only deposited on the first material encountered by the bullet during its flight. In addition, we analysed the effectiveness of SEM EDS in matching 12 different bullets to 24 bullet wipe samples, with the technician blinded to the results.

Method

White plain printer paper was placed on a polystyrene block 5-cm thick, with another paper freestanding 1 m behind it. The papers were shot from a distance of 10 and 11 m respectively, from a number 3 Enfield pressure housing fitted with appropriate barrels. Twelve different bullets were used (Table 1). Each shot was filmed with a high-speed video camera (Phantom V12; 40,000 fps), backlit to allow visual analysis of any associated air-bound gunshot residue.
Table 1

Bullets used in this study with their composition and correlated with the sample results

Bullet

Composition

Sample

Sample match by ‘blinded’ technician

Z

7.62 × 51mm L2A2

Cu 93 % Zn 7 %

1

Cu 93 %, Zn 7 %

1, 2, 5

Radway Green

 

2

Cu 93 %, Zn 7 %

 

A

7.62 × 51mm L40A1

Cu 93 %, Zn 7 %

5

Cu 93 %, Zn 7 %

1, 2, 5

Radway Green

 

6

Variable

 

W

7.62 × 51mm 93B0311

Cu 17 % Zn t-1 %

3

Cu 26 %, Zn 4 %, Sn 70 %

3, 4

DAG

Sn 81 %

4

Cu 28 %, Zn 8 %, Sn 64 %

 

D

7.62x39mm 122gr

Cu 95 %, Zn 5 %

7

Variable

8

Wolf

Fe t

8

Cu 95 %, Zn 5 % Fe t

 

Q

7.62 × 39mm Type 56

Cu 90 % Zn 10 %

9

Cu 91 %, Zn 9 %, Fe t

9, 10

Chinese 71

Fe t

10

Cu 90 %, Zn 9 %, Fe 1 %

 

T

7.62 × 39mm BXN

Cu 98 %, Zn 2 %

11

Cu 98 %, Zn 2 % Fe t

11

Czech

Fe t

12

Variable

 

V

5.56 × 45mm L2A2 NATO Ball

Cu 95 %, Zn 5 %

19

Variable

20, 21, 22

Radway Green

 

20

Cu 95 %, Zn 5 %

 

E

5.56 × 45mm 62gr

Cu 95 %, Zn 5 %

21

Cu 95 %, Zn 5 %

20, 21, 22

Federal® Tactical® Bonded®

 

22

Cu 95 %, Zn 5 %

 

G

5.56 × 45mm M193

Cu 92 %, Zn 8 %

23

Cu 92 %, Zn 8 %

23, 24

Buchsenpatronen

 

24

Cu 92 %, Zn 8 %

 

B

9 × 19mm FMJ 115gr Luger

Cu 74 %, Zn 25 %

17

Cu 75 %, Zn 25 %

17, 18

Sellier & Bellot

 

18

Cu 75 %, Zn 25 %

 

R

9 × 19mm DM11A1B2

Cu 90 % Zn 10 %

13

Cu 90 %, Zn 10 %

13, 14

Luger DAG

 

14

Cu 90 %, Zn 10 %

 

S

9 × 19mm 126gr Luger

Cu 94 %, Zn 6 %

15

Cu 94 %, Zn 6 %

15, 16

PMC®

 

16

Cu 94 %, Zn 6 %

 

After each shot, the papers were individually sealed in polyethylene sleeves and randomly allocated a number between 1 and 24. A bullet of the same make, manufacturer and batch as the bullet fired was obtained and sealed in an airtight sterile container and allocated a random letter. These were then transported to the scanning electron microscope laboratory, where the bullet holes were cut from the paper and mounted on aluminium stubs using double-sided carbon tape. All paper samples were coated with approximately 10 nm of carbon (Emitech K575X Peltier-cooled high-resolution sputter coater (EM Technologies Ltd, Kent, England)) and imaged using a JEOL JSM-6700F field emission scanning electron microscope (JEOL Ltd, Tokyo, Japan) fitted with a JEOL 2300F EDS system (JEOL Ltd, Tokyo, Japan). Areas of the sample were imaged using the LEI (lower secondary electron detector) for surface morphology and COMPO (backscatter electron detector) for chemical composition. The samples were further analysed using EDS with at least 10 areas per sample viewed and multiple particles in each area analysed. Spot analysis was used on individual particles and each spot was analysed for 100 s. The technician performing SEM EDS was blinded to all samples and was then asked to match the paper samples to those of the bullets.

Results

A total of 12 bullets and 24 samples were analysed. On the video recordings, there was no evidence of aerial gunshot residue seen around the paper samples with any of the bullets used. EDS analysis of the bullets showed that they mostly differed in composition, except for bullets Z, A, V and E. The bullets Z and A were produced by the same company (Company RG) with a composition of 93 % copper and 7 % zinc, whereas bullets V and E were produced by different companies (The British Company RG and the American Federal® Tactical® respectively), but with the same elemental composition of 95 % copper and 5 % zinc.

Initial imaging of the paper samples revealed that the area around the bullet hole contained many particles, but that bullet particles tended to be larger, brighter and more irregularly shaped. Other particles present within the bullet wipe tended to be smaller and more regular in shape, with different elements, or ratios of elements, than the bullet micro-fragments (Figs. 1, 2, 3, and 4).
Fig. 1

Overview of the edge of a bullet hole. Backscatter imaging shows bright particles against the ‘greyer’ paper background. Brightness indicates higher elemental number than the surrounding paper. Most of these particles appear to be gunshot residue rather than bullet micro-fragments

Fig. 2

Higher magnification of the area around the bullet hole. Note that some particles appear slightly larger, brighter and more irregular. These represent the bullet micro-fragments

Fig. 3

High magnification of unused plain paper

Fig. 4

EDS elemental mapping of bullet D (7.62 × 39mm 122gr Wolf), its corresponding paper sample (7) and plain paper. Note the difference in the y axis scale with the counts varying from 0–8,800 for the bullet, 0–15,000 for the sample and 0–8,000 for the plain paper

Four out of the 24 samples (6, 7, 12 and 19) were unable to be accurately assessed for elemental composition, because they had micro-fragments with compositions that differed slightly. Due to this, they could not be reliably matched to a sample. Of the remaining samples, all were correctly assigned to the bullet that was used, except for those samples shot by bullets with the same elemental composition (Z and A, V and E). In these samples, the elemental composition of each was all correctly identified, but it was impossible to accurately match the samples to the bullets with this technique (Table 1).

The highlighted bullets (Z, A, V and E) were composed of the same elemental percentages. Samples 6, 7, 12 and 19 had particles with differing compositions and were thus denoted ‘variable’ (t indicates a trace amount (less than 1 %) was present).

Discussion

A projectile fired at a paper target will leave a classical hole fringed by an area of residual debris. It is this area that forms the focus of the present study. Bullet wipe is stated to only occur with certain bullet designs and to only occur on the first target a bullet hits such as the first layer of clothing or the entry site of a wound, but not on secondary targets, deeper layers of clothing, the exit site, or any re-entry sites [3]. It is known to consist of debris from the barrel, principally soot, but also lubricant, propellant and primer by-product, as well as metal debris from the barrel itself or previous bullets [11]. However, it is not believed to contain fragments off the infringing bullet itself [3]. The darkening of the bullet wipe pattern, seen with a recently cleaned barrel as it fires more rounds, supports this idea [3]. It is thought that the bullet collects debris as it passes down the barrel and then has it wiped off as it traverses the target, leaving a ring of material around the entry hole [12]. The pattern of deposition of this material has been used to identify angle of impact of the bullet, its distance from the firearm and whether any intermediary targets were hit [13, 14, 15]. However, despite SEM EDS being extensively used to determine the bullet composition from retained bullet fragments [9], only one documented case of its use on bullet wipe to determine bullet composition has been reported [10]. This may be due to previous studies on bullet wipe raising concerns about its reproducibility and reliability [13, 14, 15]. However, all of these studies analysed the patterns of bullet wipe to reconstruct the crime scene, rather than assessing it to determine the bullet composition.

In this study, we analysed bullet wipe for retained bullet particles and found microscopic fragments of the infringing bullet dispersed within the bullet wipe up to 11 m from the barrel. We also found bullet wipe containing similar fragments on all secondary targets, dispelling the belief that bullet wipe is isolated to the primary target [3]. We found no evidence of aerial dispersion of this material, consistent with Krishnan’s results [16], and thus conclude that these fragments must have come directly from the bullet. Furthermore, because we used the same barrel for all bullets of the same calibre and were still able to correctly identify the infringing bullet, we believe that the bullet fragments in bullet wipe are not principally from previous bullet debris within the barrel, but rather, the infringing bullet itself.

However, in 4 of the 24 samples, a number of micro-fragments with slightly different elemental compositions were found, and therefore, these samples could not identify the infringing bullet with certainty. This occurrence may have been due to the micro-fragments being covered by other particles, such as gunshot residue. But, more importantly, this variance in elemental composition may represent contamination by previous bullets shot down the same barrel, which has been collected by the infringing bullet with other barrel debris and deposited as bullet wipe. Because of this, the authors advise that bullet wipe samples undergo multiple spot tests and analysis of multiple micro-fragments to reduce the inaccuracies of this technique. Furthermore, if samples give varying elemental results on the different spot tests within each sample, then these samples should be excluded.

It is known that gunshot residue (GSR) is deposited throughout the bullet’s path in contact gunshot wounds and that GSR can be used to estimate the firing distance of intermediate gunshots; however, further research is required to confirm these parameters for bullet wipe analysis [17, 18]. Furthermore, GSR can be identified in decomposed and even cremated remains, but again, further research is required to confirm this for bullet wipe [19, 20]. Finally, because we only studied full metal-jacketed bullets, only the metal composition analysis of jacket fragments is supported by this study. In addition, further studies need to confirm these results in environmentally friendly heavy-metal free ammunition [21].

Conclusion

Bullet wipe occurred with all bullet designs tested and was not isolated to the primary target, and thus, secondary targets should be analysed. In addition, this bullet wipe contained micro-fragments of the infringing bullet itself and the results of this study support the use of SEM EDS in determining bullet composition from bullet wipe, but reveal its limitation in distinguishing between bullets of the same composition. Furthermore, bullet wipe samples should be analysed with multiple spot tests to reduce inaccuracies and eliminate samples with varying elemental findings.

Notes

Acknowledgments

The authors would like to thank Glynny Kieser for her editorial input.

Ethical standards

This research complied with the current laws of the United Kingdom and New Zealand.

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer-Verlag Berlin Heidelberg 2013

Authors and Affiliations

  • D. C. Kieser
    • 1
  • D. J. Carr
    • 2
  • L. Girvan
    • 3
  • S. C. J. Leclair
    • 4
  • I. Horsfall
    • 2
  • J. C. Theis
    • 5
  • M. V. Swain
    • 6
  • J. A. Kieser
    • 6
  1. 1.Surgical Sciences, Orthopaedic Surgery, Health Sciences, Dunedin School of MedicineUniversity of OtagoDunedinNew Zealand
  2. 2.Impact and Armour Group, Department of Engineering and Applied Science, Cranfield Defence and SecurityDefence Academy of the United KingdomShrivenhamUK
  3. 3.Otago Centre for MicroscopyUniversity of OtagoDunedinNew Zealand
  4. 4.University of AngersAngersFrance
  5. 5.Surgical Sciences, Orthopaedic Surgery, Health Sciences, Dunedin School of MedicineUniversity of OtagoDunedinNew Zealand
  6. 6.Sir John Walsh Research InstituteUniversity of OtagoDunedinNew Zealand

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