Polymer Replication of Bullets & Cartridge Casings

Technical Staff: T. Brian Renegar, Alan Zheng, Mike Stocker, Robert Thompson

A polymer replication process has been developed to produce replications of bullets & cartridge casings.

These are used to generate large proficiency testing sets and share evidence between jurisdictions where

chain-of-custody requirements need to be maintained. NIST is also using the replication process to replenish

the SRM 2460 Standard Bullet used for quality control of ballistic imaging.

Polymer replication is a two-step process where: 1) A negative mold of a master bullet or casing is

produced by immersing in silicone and allowing to cure, and 2) Positive replica(s) are made from the negative

mold using polyurethane.

• A negative mold of the master Bullet Replication

bullets is produced using a 

replication container

• Vacuum/degassing is used to 

remove micro-bubbles from 

the silicone before curing

Master bullets mounted in replication container

Silicone mold and its respective Master bullets

Replicas ready to be 

removed from mold

Polyurethane replica with 

removal stub

99.37% CCF

Topography measurements of the bullet striations are performed and Cross Correlation analysis 

is performed to quantify the degree of similarity between master and replicated bullet

Polyurethane bullet replica

The polyurethane replicas are metal coated with Nickel, Chrome, or Gold using a sputter coating process.

The metal coatings give higher durability and good optical properties for microscopy. NIST has performed

testing of the replicated bullets and casings to quantify the degree of similarity of the replicas to the masters

they are produced from. The surface topography is measured using surface profiling and non-contact (optical)

techniques, and cross-correlation algorithms are used to quantify the similarity.

• Polyurethane is used to 

produce positive replicas 

from the silicone molds

• Pressurization is also used 

during curing to remove 

micro-bubbles

• Sputter coating of metals to 

replica surfaces

• Nickel or Chrome is applied 

for durability

• Gold is applied for optical 

properties and non-oxidizing Sputter coating a bullet replica

Gold coated replica on a sample holder

Optical comparison of the same land 

impression from a master bullet (left) 

and the polyurethane replica (right)

Stylus profiling 

measurement of 

bullet replica

Surface profiles from 6 lands

Cartridge Case Replication

• A similar process is used for casing 

replication

• Polyurethane replicas are produced 

from a silicone mold

• Different replication containers are 

used

• Back face of casing is replicated 

where most of the important 

features reside

Casings mounted in replication container Silicone mold and respective casings

Master casing (left) and replica (right) Comparison microscope image of a 

master casing (left) and replica (right)

Standard Bullet – SRM 2460-038

Surface topography map of Breech 

Face of replica casing

High mag comparison microscope image 

of a master casing (left) and replica (right)

• Surface topography 

measurements of casing 

surfaces of interest are 

performed (Breech Face, Firing 

Pin, Ejector)

• Area Cross Correlation 

analysis

ACCF Correlation of master casing to replica

99.05%

ACCF

• Production of a 

large proficiency 

testing set in 

collaboration with 

the Netherlands 

Forensic Institute 

(NFI) is currently 

underway

Large set of casings for replication Several of the casing molds One set of replica casings

• 162 casings x 25 molds = 4,050 total replicas to be produced

CCF Correlation analysis program

Measurement 

of casing