An Improved Vacuum Casting Method for the 

Replication of Reference Bullets

Thomas Brian Renegar, Robert M. Thompson, Alan Zheng, 

Theodore Vorburger, John Song, Johannes Soons, James Yen 

Semiconductor & Dimensional Metrology Division

Law Enforcement Standards Office

February 21, 2014
Outline

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 2

• Introduction

• Motivation

• Casting procedure overview

• Validation of process - correlation analysis

• Decay factor study & durability testing

• Possible evidentiary uses 

Blah
Introduction

•Casting of impressions, toolmarks, and 

firearm surfaces is employed as a means 

to transport evidence and preserve 

surface features.

•Casts are routinely used as primary 

evidence for analysis and comparison 

where direct examination would be 

impractical or impossible.

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 3

Blah
Introduction

• The NIST SRM 2460 Standard Bullet was 

developed to be used as a quality control 

standard in forensic laboratories

• The bullet surfaces are well characterized &

validated using surface profile analysis.

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 4

Blah
Motivation

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 5

• A total of 40 NIST SRM2460 Standard 

Bullets were produced. 

• Due to the complex manufacturing 

process, they are expensive ($2120 ea.) 

and time consuming to manufacture. 

• Almost sold out. 

Blah
Motivation

• Need an inexpensive and less time consuming method to 

replenish the Standard Bullet. 

• Requirements: 

 Needs to retain the same surface topography 

quality as the original standard bullets 

 Color/translucency properties must be compatible 

with microscope imaging

 Durable

Polymer replication using vacuum casting technique could 

potentially be used to restock the Standard Bullets

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 6

Blah
European Research

“Vorrichtung zum Abformen von Hülsen und 

Geschossen unterschiedlicher Kaliber”

Alfons Koch, 2010

(Patent application DPMA DE 10 2005 039 

823.5-15)

BKA/NIST signed MOU in 2011

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 7

Blah
Vacuum Casting Technique

- Replication container

A replication container was fabricated to house the master 

bullets during the silicone molding phase. 

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 8

Blah
9

Mixing silicone Vacuum-degassing in desiccator

Blah

Vacuum Casting Technique

- Step 1: Silicone Mold 

Pouring silicone into replication rig 2

nd vacuum-degas
Vacuum Casting Technique

- Removing the silicone mold from the replication rig

10

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Vacuum Casting Technique

- Step 2: Polyurethane Replica

11

Blah

Mix two-part polyurethane and coloring dye. Then vacuum-degas.

Use a dropper to fill mold with polyurethane. Vacuum-degas again and let cure.
Vacuum Casting Technique

- Removing the cured polyurethane replica bullets from 

the silicone mold

12

Carefully separate the silicone from the urethane replicas and then 

remove the replicas with needle nose pliers. The bullet standoff in 

the replication rig will avoid contact with the striated regions of the 

bullets, avoiding any damage. 

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
First Replication

(before vacuum degassing was implemented)

13

Silicone Mold

Urethane Replica

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
First Replication

(before vacuum degassing was implemented)

14

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

(5X Optical Image)

Quality control of micro-bubbles needs to be addressed
Improvements to process

15

• Vacuum / degassing 

using a belt driven 

“roughing” pump during

mixing process

5 x 10-2 Torr

(6.5 x 10-5 atmosphere)

• Changes to silicone & polyurethane materials

 Reduced viscosity (pours better)

 Longer working time before curing

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Improved Procedure

16

(5X Optical comparison)

 Replica from Mold # 13a Original SRM 2460-038

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Blah

17

Close-up images of 

Standard Bullet 

replicas

SRM 2460-038
Blah

Stylus Measurement of the Replica Bullets

18
Blah

Example of CCF Correlation Program

19

VERY High CCF value of 99.37% indicates that the 

Replica is virtually identical to the Master Bullet

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Stylus profile comparison of virtual signature to Replica

99.37%
Blah

Decay Factor: Time 

20

Measurement Date CCFmax % Lateral Scaling

5/29/2012 99.37 1.0055

5/30/2012 99.41 1.0060

5/31/2012 99.47 1.0050

6/1/2012 99.50 1.0055

6/4/2012 99.39 1.0055

6/5/2012 99.41 1.0050

6/6/2012 99.45 1.0050

6/7/2012 99.46 1.0055

6/11/2012 99.48 1.0055

6/12/2012 99.51 1.0055

...

11/26/2012 99.41 1.0050

...

02/14/2014 99.44 1.0055

Mold 10, Bullet # 15 – Consecutive

measurements vs. Virtual Standard

High CCF% 6 months 

& 1 1⁄2 years later
Decay Factor: Sibling Replications 

21

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Multiple replicas made from

a single mold.
Decay Factor: Sibling Replications - CCF Results

22

Blah

Sibling Replication # CCFmax % Lateral Scaling

1 99.30 1.0045

2 99.32 1.005

3 99.41 1.005

4 99.16 1.005

5 99.20 1.0045

6 99.25 1.003

7 98.72 1.004

8 98.22 1.004

9 98.38 1.0045

10 97.98 1.0045

11 97.45 1.005

12 97.39 1.005

13 97.17 1.0045

14 97.09 1.0045

15 93.42 1.0035

Replicas from Mold # 9, Bullet 038, Land 1
23

2460-038

Master

Mold #9

1

st Replica

Mold #9

3

rd Replica

Mold #9

6

th Replica

Mold #9

9

th Replica

Mold #9

12th Replica

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Sibling Replications – Optical Comparisons
Decay Factor: Generation Test

24

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Each replica is used to create a new mold

Master Bullet #’s 17, 32, & 38 used to create first mold (13a)
Decay Factor: Generation Test

25

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Close-up of Generation Replicas

From Master Bullet # 038

Mold 13a Mold 13b Mold 13c Mold 13d

Note: Standoff in replication rig reproduces itself during each casting cycle
Decay Factor: Generation Test – CCF Results

26

Generation # CCFmax % Lateral Scaling

1 (Mold 13a) 99.59 1.004

2 (Mold 13b) 99.41 1.0095

3 (Mold 13c) 99.45 1.014

4 (Mold 13d) 99.43 1.019

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Note: Each urethane replica shrinks by 0.4 - 0.5%.

This is a compounding effect from generation to generation.

Replicas from Molds 13_, Bullet 038, Land 1
Generation Test – Optical Comparisons

27

2460-038

Master

Mold #13a

(1

st Generation)

Mold #13b

(2

nd Generation)

Mold #13c

(3

rd Generation)

Mold #13d

(4

th Generation)

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Durability Testing

28

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Real world “stress tests” are conducted on polymer 

replica bullets to ensure their durability, and suitability to 

be used as reference masters.

• High Temperature – Replica # M15-38-1 heated to 55oC 

(130oF) for 3.5 hours

• Low Temperature – Replica # M15-32-1 cooled to -12oC 

(10oF) for 8 hours

• Handling – Replica # M15-38-3 handled with bare hands on 

land impressions and dropped 1.5 meters to hard surface 10 

times

• Chemical – Replica # M15-38-2 immersed in Ethyl Alcohol 

for 20 minutes
Durability Testing

29

Test Type Control CCF% 

(Before Test) After Test CCF%

High Temperature 99.42 99.51

Low Temperature 99.36 99.42

Handling 99.48 99.35

Chemical 99.37 98.74

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Correlations compared to Virtual Standard

Replica bullets are measured/analyzed before and after 

“stress tests” using stylus profilometer
Durability Testing

30

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

High Temperature (replica # M15-38-1) 

Low Temperature (replica # M15-32-1)

Before

After

Before

After
Durability Testing

31

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 

Handling & drop test (replica # M15-38-3)

Chemical immersion (replica # M15-38-2)

Before

After

Before

After
Possible Evidentiary Uses

• Inter-laboratory transfer of bullet (and 

cartridge case) evidence for comparisons.

Logistics and chain of custody issues with 

transferring actual evidence are alleviated

• International evidence transfer.

• Large proficiency test production

• Eliminates sample variation in production runs. 

• Pre-evaluated samples representative of 

casework difficulty can be produced.

32

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Future Work

• Improve Bullet Replicas

 Testing of release agents, hydrophobic 

coatings, etc. to reduce mold tearing from 

successive replications. -currently underway

 Utilize pressurization in conjunction with 

vacuum-degassing during silicone/urethane 

curing.

 Continue durability testing of polymer replicas 

(include abrasives, oil, humidity, etc.)

• Replication of Cartridge Cases

33

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Acknowledgements

The funding for this research was partly 

provided by the U.S. National Institute 

of Justice (NIJ) through the Office of 

Law Enforcement Standards (OLES) at 

NIST.

34
T. Brian Renegar

National Institute of Standards and Technology

100 Bureau Drive, Mail Stop 8212

Gaithersburg, MD 20899-8212

(301) 975-4274

brenegar@nist.gov

35

Thank you!

Questions?

Blah

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 
Previous Research

• “The Production of Replicas of Bullets and Cartridges”, by Geradts, Kreijzer, & C. Van Brakel, 

Netherlands Forensic Science Laboratory.

• Developed procedures to counter shrinkage effects using modern casting materials.

• Potential transfer of evidence in EU.

Reference: AFTE Vol 28, No1 Jan 1996 

• “Casting of Complex Stereometric Samples for proficiency Tests in Firearm & Toolmark

Examinations” Kock & Katterwe, BKA Germany.

• Technique described in the production of a large inter-laboratory proficiency test for bullet identification.

• Evaluated new materials and adapted procedures.

Reference: AFTE Vol 39 No4 Fall 2007

• “Topography measurements for determining the decay factors in surface replication” Song, 

Rubert, Zheng & Vorburger; NIST.

• Developed procedures to test and measure the decay factor in the replication of surface topography.

Reference: Proceedings of ISMTII 2007 MST11

• “Topography Measurements and Performance Comparisons between NIST SRM 2460 Standard 

Bullet Masters and BKA Bullet Replicas”, Song, J., Vorburger, T.V., Thompson, R., Ballou, S., 

Zheng, A., Renegar, T.B., Silver, R., Ols, M., W. Wenz, A. Koch, M. Braune, A. Lohn, AFTE 

Journal, 44, 3, 2012, pp.201-217.

SEMICONDUCTOR & DIMENSIONAL METROLOGY DIVISION | LAW ENFORMCEMENT STANDARDS OFFICE 36

Blah