United States Department of Agriculture / Forest Service

Rocky Mountain Research Station

Research Paper RMRS-RP-104

August 2013

A Study of Ignition by Rifle Bullets

Mark A. Finney

Trevor B. Maynard

Sara S. McAllister

Ian J. Grob
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Finney, Mark A.; Maynard, Trevor B.; McAllister, Sara S.; Grob, Ian J. 2013. A study of ignition by rifle 

bullets. Res. Pap. RMRS-RP-104. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, 

Rocky Mountain Research Station. 31 p.

Abstract

Experiments were conducted to examine the potential for rifle bullets to ignite organic matter after 

impacting a hard surface. The tests were performed using a variety of common cartridges (7.62x51, 

7.62x39, 7.62x54R, and 5.56x45) and bullet materials (steel core, lead core, solid copper, steel 

jacket, and copper jacket). Bullets were fired at a steel plate that deflected fragments downward 

into a collection box containing oven-dried peat moss. We found that bullets could reliably cause 

ignitions, specifically those containing steel components (core or jacket) and those made of solid 

copper. Lead core-copper jacketed bullets caused one ignition in these tests. Ignitions of peat also 

occurred with a small set of tests using solid copper bullets and a granite target. Thermal infra-red 

video and temperature sensitive paints suggested that the temperature of bullet fragments could 

exceed 800°C. Bullet fragments collected from a water tank were larger for solid copper and steel 

core/jacketed bullets than for lead core bullets, which also facilitate ignition. Physical processes are 

reviewed with the conclusion that kinetic energy of bullets is transformed to thermal energy by plastic 

deformation and fracturing of bullets because of the high-strain rates during impact. Fragments cool 

rapidly but can ignite organic matter, particularly fine material, if very dry and close to the impact site.

Keywords: bullet fragmentation, fire ignition, shooting ignition 

The Authors

Mark A. Finney: USDA Forest Service, Missoula Fire Sciences Laboratory, 5775 Highway 

10 West, Missoula, MT 59808. mfinney@fs.fed.us, 406.329.4832

Trevor B. Maynard: USDA Forest Service, San Dimas Technology and Development Center, 

444 E. Bonita Ave. San Dimas, CA 91773. tbmaynard@fs.fed.us, 909.599.1267 x258

Sara S. McAllister: USDA Forest Service, Missoula Fire Sciences Laboratory, 5775 Highway 

10 West, Missoula, MT 59808. smcallister@fs.fed.us, 406.329.4907

Ian J. Grob: USDA Forest Service, Missoula Technology and Development Center, 5785 

Highway 10 West, Missoula MT 59808. igrob@fs.fed.us, 406.329.6775
Contents

Introduction.............................................................................................................. 1

Physical Processes................................................................................................. 2

Ignition of Organic Matter ............................................................................. 2

Firearms and Energy Conversion................................................................. 3

Heating Mechanisms............................................................................................... 3

Impact Mechanics......................................................................................... 5

Kinetic Energy—a Conceptual Model........................................................... 5

Stress/Strain-Based Models......................................................................... 7

Post-Impact Heat Losses ............................................................................. 7

Heat Loss from Radiation............................................................................. 8

Experimental Methods ............................................................................................ 9

Fragment Temperatures ..............................................................................11

Statistical Analysis...................................................................................... 12

Videography ............................................................................................... 12

Results.................................................................................................................... 13

Bullet Material............................................................................................. 14

Temperatures of Bullet Fragments ............................................................. 21

Discussion ............................................................................................................. 22

Acknowledgments................................................................................................. 25

References ............................................................................................................. 26

Appendix ................................................................................................................ 27

USDA Forest Service Res. Pap. RMRS-RP-104. 2013 1

Introduction

In the United States, outdoor target shooting has been 

suspected as the source of numerous wildland fires1,2. 

Anecdotally, the ammunition involved in most incidents 

is thought to be of ordinary commercial varieties with 

bullets composed of inert materials including lead, 

steel, and copper. No scientific studies have specifically addressed projectile behavior or properties related 

to ignition of wildland vegetation or organic material. 

Thus, the primary focus of this study is whether inert 

projectiles fired from commonly available modern rifles 

can cause ignition of wildland vegetal matter.

The possible mechanism by which inert projectiles 

could cause ignitions involves the conversion of kinetic 

energy to thermal energy at impact with a solid object 

or target. Kinetic energy is proportional to the product 

of an object’s mass and square of velocity, which is 

well known for the vast variety of cartridges available 

for modern firearms. In general, pistol cartridges are 

designed to propel a bullet much slower with less energy 

than rifle cartridges. Rifle bullets should thus be the most 

likely to have sufficient energy for ignition, provided 

some amount of that energy is converted to heat after 

impact. Table 1 indicates approximate muzzle energy 

for a variety of different cartridges.

Ballistic impact has been researched extensively but 

has been directed principally toward understanding 

penetration or perforation of target materials. For a particular target, projectiles of a given speed will perforate 

it at higher angles (closer to normal) and ricochet at 

lower angles (more oblique) (Johnson and others 1982, 

1 Target shooting starts brush fire near Saratoga Springs. http://

www.newsutah.org/utah/target-shooting-starts-brush-fire-14998/

2 http://content.usatoday.com/communities/ondeadline/

post/2012/07/gunfire-blamed-for-some-wildfires-states-considerlimits/1#.UCF4202PXwk

Goldsmith 1999). The consequences of such impacts 

most pertinent to ignition are where the impact:

1. Converts a large fraction of kinetic energy to thermal energy, 

2. Fractures the bullet or target into pieces large 

enough to ignite organic matter, and 

3. Ejects hot material into the organic matter. 

We reasoned that these conditions should occur most 

commonly with oblique impacts on a highly resistant 

target (no penetration or perforation). This would produce sufficient plastic deformation and friction of the 

projectile and direct the ricochet of particles into nearby 

organic wildland substrates. Plastic deformation is the 

irreversible change in shape of a material caused by an 

applied mechanical force. For most metals, upwards of 

90% of the energy dissipated during rapid plastic deformation is manifested as heat (Rogers 1979, Yildirim 

and others 2011), which is maintained within the bullet 

fragments for some period of time after impact. 

No empirical studies of bullet fragment temperatures 

have been found, but numerical experiments have shown 

temperature at the impact interface to increase with 

velocity (Molinari and Ortiz 2002, Yildirim and others 

2011). At velocities comparable to rifle bullets (700-

1000 m s

-1) (Table 1), modeled temperatures of impact 

surfaces exceed 500 °C (Yildirim and others 2011, 

Molinari and Ortiz 2002). Additionally, laboratory 

studies of plastic deformation heating at less than ballistic velocities have yielded temperature rises exceeding 

350 °C (Hartley and others 1986). The ability of metal 

pieces to cause ignition was studied using heated steel 

spheres (Hadden and others 2011) and showed that hotter 

and larger spheres were more likely to cause ignitions in 

cellulose. The study of particles from arcing power lines 

(Tse and Fernandez-Pello 1998) suggests that smaller 

bullet fragments ejected from the impact site can fly 

farther but lose temperature more quickly than larger 

pieces. 

Table 1. Typical velocity and kinetic energy of common small arms cartridges (U.S. Army 1994). Note: 1lb = 7000 grains.

 Weight, g Muzzle velocity, Kinetic energy, J

Cartridge Typical firearm used (grains) m s-1 (ft s-1) (ft-lb)

.22LR 22 rifle 3 (40) 361 (1185) 195 (144)

.45 ACP M1911 Semi-automatic pistol 15 (230) 270 (885) 547 (404)

5.56 x 45mm NATO M-16 rifle 4 (62) 920 (3025) 1693 (1249)

7.62 x 51mm NATO M-14 rifle 9 (146) 840 (2750) 3175 (2343)

.50 BMG M2 heavy machine gun 43 (660) 850 (2800) 15533 (11464)
2 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Physical Processes

A series of physical processes are involved in transforming energy from the moving bullet into energy available for ignition. For a material to be ignited, it must be 

heated to a temperature where oxidation reactions of the 

material release enough heat to independently continue 

the process. In the present situation, metal particles must 

be capable of contacting the fuel and conducting heat to 

it long enough to raise the temperature at contact points 

high enough for the oxidation reactions to self-sustain. 

The exact temperature at which this occurs is unknown 

and depends on the fuel, fuel bed characteristics, and 

environmental conditions. Because metal bullet fragments are small, more contact is likely with relatively 

fine grained material, which both increases likelihood 

of contact and decreases heat losses to adjoining material after contact. Water content changes the thermal 

properties of the fuel such that drier fuels require less 

heating to raise temperatures at the contact point to the 

critical level. 

The size and temperature of bullet fragments resulting from impact depends very much on the mechanical 

properties of their constituent materials. The pertinent 

properties determine the response to rapid deformation 

and fracturing due to the high speed of bullets (~900ms

-1

or ~3000 ft s-1). Forces within the bullet at impact are 

responsible for compressing and shearing the metal 

into smaller pieces. Materials like steel or copper that 

require high energy to deform them (due to a greater 

“toughness”) are known to release this energy mostly 

as heat. Materials like lead are more easily deformed 

and melt at low temperatures and thus contain less heat 

and fracture in smaller pieces. The size of the pieces is 

important to both the distance they ricochet from the 

target and the rate at which they cool, mostly by contact 

with the cool air (convection). Tougher metals break into 

fewer and larger pieces and thus cool more slowly than 

smaller particles. 

Although the data available on fragment temperatures 

after impact are limited, rough calculations indicate that 

bullet energy should be capable of raising bullet temperatures above 1000 °C (1832 °F) under ideal conditions. 

The mechanical properties of metals vary considerably 

and will affect the actual temperature rise but are not 

well understood. Even so, the physical processes strongly 

demonstrate that ignition should be possible from rifle 

bullets and will vary by bullet material. A description of 

each of these physical processes as pertaining to bullet 

fragment ignition is discussed below.

Ignition of Organic Matter

Piloted ignition is the most common cause of wildland 

fires. Typical human-caused ignition sources include 

embers from campfires, arcing power lines, and hot 

debris from cutting and grinding (NWCG 2005). In 

each of these cases, the activity produces fragments 

of hot material that are capable of transferring enough 

heat to ignite surrounding vegetation. As mentioned in 

the introduction, in some cases, bullet impacts appear 

to generate high temperature fragments that could act 

as pilot ignition sources. 

To explore the mechanism by which a bullet impact 

may lead to ignition of nearby vegetation, we first discuss the fundamentals of the ignition process. Ignition 

of wildland fuels can occur in two modes: flaming or 

smoldering. For thorough reviews of these modes of 

combustion, see Babrauskas (2003), Torero (2008), 

Ohlemiller (2008), and Rein (2009). Flaming ignition 

occurs when the fuel is exposed to a heat source that 

provides sufficient energy to cause thermal decomposition of the material (pyrolysis). The gaseous pyrolysis 

products must escape the solid and mix with the air 

(oxidizer). This mixture can either be ignited with an 

external localized energy source (piloted ignition) or 

ignited via self-heating (autoignition). To achieve sustained flaming ignition (and hence establish a diffusion 

flame), the rate of decomposition (pyrolysis) must be 

sufficiently large so that the heat release rate from this 

premixed fuel-air flame is larger than the heat loss rate. 

In contrast to flaming ignition, which occurs in the gas 

phase, smoldering ignition occurs in the solid phase and 

no visible flame is seen. Smoldering is also initiated by 

heating the solid to a temperature high enough for thermal 

degradation (pyrolysis) to occur, resulting in formation 

of a char. The required heating rate is typically much 

slower than for flaming ignition. If available, oxygen will 

react directly on the surface of the solid char left by the 

pyrolysis reaction. This oxidation reaction is exothermic, 

generating all the heat from smoldering combustion, and 

is the heat source for further propagation of smoldering 

combustion. Smoldering combustion is thus a much 

slower form of combustion with considerably lower 

temperatures than flaming combustion (approximately 

600 °C versus 1200 °C). However, the heating required 

to initiate smoldering combustion is considerably lower 

than flaming combustion. Additionally, smoldering 

can transition to flaming combustion when conditions 

change, such as more air (oxidizer) becoming available 

when the wind speed increases, creating a “hazardous 

shortcut to flaming fires” (Rein 2009). In forest fuels, 
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 3

this transition from smoldering to flaming can take days, 

even weeks, such as in lightning ignited fires (Wotton 

and Martell 2005, Pineda and others 2012). Smoldering 

ignition is thus as much of a concern as direct flaming 

ignition.

This study is focused on the likelihood of ignition 

of wildland fires via contact with hot bullet fragments. 

Unfortunately, ignition by contact with hot particles is 

not well understood (Babrauskas 2003, ch. 7, 11, and 14). 

There are relatively few well-controlled experimental 

studies examining this mode of ignition and even fewer 

practical theoretical models. According to Babrauskas 

(2003), existing theoretical models fall short for four 

main reasons: no experimental validation, most practical 

situations violate the basic assumptions of the model, 

the problem is probabilistic and no models include 

probability, and the material properties of the solid to 

be ignited are largely unknown (page 500). Based on 

what little research has been done in this area, however, 

a few general trends have been noted. In general, the hot 

particle must be at a substantially higher temperature 

than the ignition temperature measured under radiant or 

convective heating (Setchkin 1949, Kuchta and others 

1969, Tanaka 1977). Additionally, higher temperatures 

are required to initiate both smoldering and flaming 

ignition as the particle size decreases (Hadden and others 2011, Rowntree 1994, Stokes 1990, Gol’dshleger and 

others 1973, Tanaka 1977). By performing experiments 

with steel and aluminum ball bearings in conjunction 

with a theoretical model, Gol’dshleger and others (1973) 

showed that increasing the conductivity of the hot particle 

lowered the required particle temperature. Studies with 

firebrands indicate that a higher flux of particles will 

increase the probability of ignition (Manzello and others 

2006a, 2006b, 2008). Increasing the moisture content 

of both sawdust and pine needle beds has been shown 

to increase the required particle temperature (Tanaka 

1977) and thus decrease the ignition probability (Ellis 

2000). The density or physical structure of the receptive fuel was also shown to influence the probability of 

ignition, with fluffy cotton much easier to ignite than 

tightly woven cotton (McGuire and others 1956). 

As a worse-case scenario, dry fluffy commercially 

available peat moss was chosen in this study as the 

receptive material. The density, moisture content, and 

mineral (or ash) content of peat has been shown to affect 

its ignitability. In general, it is more difficult to ignite 

peat when the density or moisture content increases 

(Hartford 1989, Grishin and others 2006). However, the 

minimum ignition energy of Russian peat was shown by 

Grishin and others (2006) to have an optimum level of 

both density and moisture content. For the lowest density tested (0.38x103

 kg m-3), an increase in the ignition 

energy was seen. A similar increase in ignition energy 

was seen with extremely low values of moisture content 

(<1% MC). The combined effect of mineral content and 

moisture content was examined by Frandsen (1987, 1997) 

where it was shown that as the mineral content increases 

the maximum moisture content for ignition decreases. 

Firearms and Energy Conversion

In physics, energy is a measure of the ability to perform mechanical work. For a firearm, the mechanical 

work being performed is the acceleration of a projectile 

to a desired velocity. Though energy can exist in many 

forms, the most relevant for firearms are potential, 

kinetic, and thermal energy. Potential energy is stored 

energy that could be converted to work. Examples of 

potential energy include a compressed spring (elastic 

potential energy) and propellant stored inside of an ammunition cartridge (chemical potential energy). Kinetic 

energy is the energy possessed by an object due to its 

motion. Thermal energy is associated with an object’s 

temperature.

The discharge of a firearm involves the exchange of 

significant amounts of energy (Figure 1). Prior to firing, potential energy is stored in the form of propellant 

inside of the cartridge. Once the trigger is pulled and 

the firing pin strikes the primer on the cartridge, the 

propellant ignites and burns rapidly. This generates hot, 

expanding gases that propel the bullet down the barrel, 

giving it kinetic energy. Typical values of kinetic energy 

for common cartridges are shown in Table 1.

Heating Mechanisms

During firearm discharge, bullets are exposed to 

several sources of heating. Inside the barrel, heat is 

transferred to the bullet by the propellant gases and by 

friction with the barrel itself. The bullet is also heated 

by friction with air as it travels toward the target at 

supersonic speed. However, in both cases, the duration 

is extremely short, so their contribution to temperature 

increase is likely minimal.

If in-barrel and aerodynamic heating are negligible in 

terms of bullet temperature increase, the only remaining 

source of heating is interaction between the bullet and 

target. As the bullet impacts its target, some or all of 

its kinetic energy is converted into other forms. Some 

is converted to observable mechanical work, such as 
4 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

deformation and perforation of the target and distortion of the bullet. Part of the kinetic energy is also 

dissipated into forms that cannot be easily observed, 

such as thermal energy (temperature) in the projectile 

and target, and pressure waves. Depending on target/

projectile construction and impact angle, the bullet may 

retain some of its kinetic energy (and thus its motion) 

after impact. 

The mechanics of energy dissipation depend on target 

and bullet construction. A paper target requires a minimal amount of energy to perforate; a bullet penetrating 

it will transfer a small amount of its kinetic energy and 

will continue traveling at a slightly reduced velocity. 

However, a more resilient target, such as a steel plate, 

requires a large amount of energy to become deformed. 

Bullet construction is also important. The various types 

of bullets used in this study are shown in Figure 2. 

Bullet cores were lead, copper, or steel. Because rifle 

bullets are subject to high muzzle velocities, they are 

typically covered with a thin layer of a protective 

metal, called a jacket. All rounds used in this study 

used steel or copper jackets. In general, depending on 

their intended use, bullets may be designed to achieve 

immediate expansion within the target (expanding bullets) or penetration through the target (armor-piercing 

rounds). Expanding bullets are intended to “mushroom” 

on impact (Figure 3). They are often used for hunting 

and other scenarios where complete penetration of the 

Figure 1—Conversion of energy from potential to thermal and kinetic energy during firearm 

discharge

Figure 2—Cross-sections of bullets used in this study. The use of tradenames is 

provided for informational purposes only and does not constitute an endorsement 

by the U.S. Department of Agriculture. Photograph by J. Kautz, USFS.
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 5

target is undesirable. Armor-piercing rounds typically 

contain a penetrator constructed of steel or another high 

density metal. The jacket is destroyed on impact, but 

the penetrator’s momentum propels it into the target 

substrate. 

Impact Mechanics

When a material is subject to sufficient mechanical 

stress, its shape begins to change (Figure 4). Initially, 

this change is reversible—if the load is removed, the 

material returns to its original form. This is called 

elastic deformation (Popov 1976). As the amount of 

applied stress increases above a critical level, however, 

it eventually leads to permanent, irreversible deformation of the material, called plastic deformation. Further 

stress will increase plastic deformation until the material 

eventually fails structurally. 

The energy expended during plastic deformation is 

converted to two forms: stored energy (changes to the 

metal’s microscopic structure), and thermal energy 

(heat) (Ravichandra and others 2002). During rapid 

(high strain-rate) plastic deformation, heat is generated 

in the material more rapidly than it is lost, leading to 

a temperature rise. For most metals, almost all energy 

conversion during high strain-rate deformation is manifest as heat (Rogers 1979, Kapoor and Nasser 1998), 

leading many investigators to consider the deformation 

to be adiabatic, meaning a negligible amount of heat 

loss takes place during the process. 

Kinetic Energy – a Conceptual Model

As a first approximation, the maximum temperature 

rise due to impact can be estimated by assuming that 

all kinetic energy is converted into plastic deformation 

heating. This approximation is crude and will yield an 

overestimate of temperature rise, since energy is also 

dissipated by other mechanisms (sound/pressure waves, 

deformation and heating of the target). 

The kinetic energy of an object is related to both its 

mass and velocity:

(1)

where KE is the kinetic energy (J), m is the mass (kg), 

and v is the velocity of the projectile (m s-1). The change 

in thermal energy due to temperature rise in a material 

is given by: 

(2)

where ∆T is the increase in temperature (K) and c

(J kg-1 K-1) is the specific heat (the amount of thermal 

energy required to raise a unit mass by one Kelvin). If 

we assume all kinetic energy goes into increasing the 

thermal energy of the material (heating), then we can 

equate (1) and (2):

(3)

Figure 3—Expanding rifle bullet. Unfired cartridge 

(left) and bullet after impact (right).

Figure 4—A typical stress-strain curve (simplified), 

which shows how a material changes shape under 

an applied mechanical load.
6 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Since mass appears on both sides of (3), it can be 

eliminated, leaving us with a simplified estimate of 

temperature rise:

(4)

Of significance in Eq. 4 is the lack of dependence 

of temperature rise on mass. For this idealized impact 

of a bullet of given material at a specified velocity, the 

temperature increase of the projectile is independent 

of its size and shape. In our experiments, all bullets 

deformed to the point of fracture, resulting in individual 

fragments rather than a single mass. All the heating 

was due to plastic deformation that occurred prior to 

fragmentation. Assuming the bullet was a uniform temperature throughout just prior to fragmentation, all the 

fragments should be the same temperature and equal 

to the temperature of the whole. 

The temperature rises predicted by Eq. 4 for monolithic projectiles of lead, copper, and steel are shown 

in Figure 5. The specific heats of lead, copper, and 

steel at 20 °C are 0.128 kJ kg-1 K-1, 0.386 kJ kg-1 K-1, 

and 0.490 kJ kg-1 K-1, respectively, but increase with 

increasing temperature. Temperatures in Figure 5 were 

calculated using the specific heat at the midpoint of 

ambient and final temperature (determined using an 

iterative procedure). As an example, the velocity of 

a Winchester 7.62x54R 180-grain soft point bullet at 

25 yards is approximately 750 m s-1 (2500 ft s-1), which 

yields temperature increases of 2233 °C (4019 °F), 

660 °C (1220 °F), and 502 °C (936 °F) for lead, copper, 

and steel projectiles, respectively. One method of validating this prediction is by observing the condition of 

recovered fragments. Lead melts near 320 °C (600 °F), 

copper near 1100 °C (2000 °F), and carbon steels between 

1400-1500 °C (2600 – 2800 °F). Some lead fragments 

Figure 5—Temperature rise versus impact velocity for an idealized projectile impact (Eq. 4). 
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 7

recovered during our experiments appeared to have 

melted and re-solidified, but this was not evident for 

any copper or steel fragments. This observation, though 

rudimentary, is in agreement with the calculated values. 

Stress/Strain-Based Models

The simple model described above does not account 

for the mechanical properties of materials subject to 

impact loading. The amount of heating caused by plastic 

deformation depends on strain rate, the type of loading, 

and the material itself. 

When a bullet strikes a target, it deforms plastically. 

With sufficient impact velocity and target rigidity, the 

bullet may fail structurally with only minimal penetration 

into the target. Xiao and others (2010) identified four 

distinct deformation and failure modes for blunt-shaped 

steel (38CrSi) projectiles fired at rigid steel plates. The 

first mechanism, mushrooming, occurred at relatively 

low velocities (150-250 m s-1 [500-800 ft s-1]) and was 

characterized by the radial expansion at the nose of the 

projectile, leading to a mushroom-shaped appearance, 

but not causing fracture. As impact velocity increased, 

the second failure mode, shear cracking, became evident. 

The mushrooming effect causes stresses to be localized 

in the head of the projectile, which can lead to cracks 

that begin at the impact face and propagate backwards. 

Of significance in Xiao and others’ (2010) experiments 

was the bluish discoloration (oxidation) at the head 

and cracked interface of the projectile, indicating significant heating. For plain carbon steels, this occurs at 

255-320 °C (500-600 °F) (Oberg and others 1990). 

After shear cracking, the failure modes observed 

by Xiao and others (2010) were dependent on material hardness. Softer projectiles experienced petalling. 

Petalling is an extension of shear cracking but occurs 

at higher velocity. The shear cracks propagate even 

farther rearward, giving the projectile a petalled appearance, similar to the expanded bullet shown in 

Figure 3. Significant discoloration of the petals was 

evident over much of their length, indicating sustained 

high temperatures during deformation. Hard projectiles 

experienced fragmentation—the projectiles shattered 

into many pieces. The number of fragments increased 

with projectile velocity. Fragments had some surface 

discoloration, but it was less prevalent than with the 

softer petalled projectiles. 

Currently, there is no accepted theoretical model 

that completely describes the heating of objects as they 

undergo plastic deformation. However, Quinney and 

Taylor (1937) proposed the use of an energy balance 

model to estimate the material temperature rise as a 

function of strain:

(5)

where η is the fraction of plastic deformation work 

converted to heat, and ∆W is the mechanical work 

done. Since the area under the true stress-strain curve 

represents the plastic work done per unit volume (for 

deformation in a single axis), the temperature rise can 

be estimated as a function of strain:

(6)

(7)

where ε is the true strain, σ is the true stress (N m-1), 

and cv is the specific heat of the material at constant 

volume (J kg-1 K-1). Though dynamic stress-strain data 

are limited for high strain rate experiments (due to 

the difficulty of measurement), we can make some 

qualitative arguments about the effect of different materials. Brittle materials have limited ductility and do 

not undergo much plastic deformation before fracturing, 

while very ductile materials (like copper) can experience significant plastic deformation (Rittel and Osovski 

2010). In general, the area under the stress/strain curve 

(the integral in Eq. 7) will increase with ductility (assuming the strength remains comparable), which predicts 

a greater temperature rise for more ductile materials. 

Post-Impact Heat Losses 

After impact, fragments are reflected from the target 

surface and travel some distance before reaching the 

ground. During flight, fragments will lose heat through 

convective and radiative heat transfer to the surroundings. 

Though their time of flight was not measured, visible 

observations indicated that fragments in our experiments 

were airborne for only a fraction of a second before 

landing on the surface. To estimate the effect of heat 

loss during this time, consider the deflection of a bullet 
8 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

fragment created during impact. As the fragment falls 

toward the ground, its surface is exposed to an external 

airflow that aids in removing heat from the material—this 

is called forced convection. Because the fragments are 

small (“thermally thin”), this situation is amenable to 

a special type of heat transfer analysis called lumped 

capacitance (Incropera and DeWitt 2007). 

Using lumped capacitance analysis, the time required 

for an object to experience a specified change in temperature is given as:

(8)

where ρ is the density of the material (kg m-3), V is 

the volume of the object (m3), h is the convective heat 

transfer coefficient (W m-2 K-1), AS is the surface area 

(m2), and subscripts i, f, and ∞ represent the initial, final, and ambient temperatures, respectively. In forced 

convection, the convection coefficient is determined 

from the Nusselt number, which represents the ratio of 

convection to conduction heat transfer:

(9)

where k is the thermal conductivity of the surrounding 

fluid (air) (W m-1 K-1) and L represents the length scale 

of the object (m). For circular cylinders in cross flow, 

Churchill and Bernstein (1977) provide a correlation for 

the Nusselt number over a wide range of flow conditions:

 (10)

where Re is the Reynolds number, the dimensionless 

ratio of inertial to viscous forces in the flow, and Pr is 

the Prandtl number, a dimensionless ratio of momentum and thermal diffusivities in the fluid. Determining 

the Reynolds number requires the ricochet velocity be 

known. Experimental data on the ricochet velocity of 

fragments is scarce, primarily due to the difficulty of 

measurement. However, intuition leads us to expect that 

it will be proportional to the impact velocity. 

Using the initial fragment temperatures obtained 

by Eq. 4, and assuming cylindrical fragments with a 

ricochet velocity of 50% of the impact velocity, the 

time required to reach 275 °C (fragments below this 

temperature would be unlikely to act as ignition pilots) 

is shown in Table 2. Since even very small fragments 

take more than 0.1 s to cool to 275 °C, it seems possible 

that fragments may reach the ground at temperatures 

high enough to transfer significant amounts of heat to 

the surface.

Heat Loss from Radiation

Fragments are also subject to heat loss through radiation. The contribution of radiative heat loss, relative to 

that of convection, can be estimated using the radiation 

heat transfer coefficient (Incropera and DeWitt, 2007)

(11)

where εf is the emissivity of the fragment, Tf is the 

fragment temperature, and σ is the Stefan-Boltzmann 

constant (5.67 x 10-8 W m-2 K-4). For the conditions assumed in Table 2, the radiation heat transfer coefficients 

Table 2—Time required for a cylindrical fragment to reach 275 °C (530 °F). 

Initial temperature estimated by Equation 4 and assuming a ricochet velocity of 50% of impact velocity, using lumped capacitance 

analysis. Lead is not shown, as it would be in a liquid phase above 

230 °C, requiring more complex analysis. 

Diameter, 

m (inch)

Length/

Diameter

Time to reach 275 °C (530 °F) (s)

Steel (Ti

 = 614 °C) Copper (Ti = 767 °C)

0.005 (0.2)

0.25 0.09 0.12

0.5 0.18 0.23

0.75 0.28 0.35

1.0 0.37 0.46

1.5 0.55 0.69

2.0 0.73 0.92

3.0 1.10 1.38

5.0 1.84 2.31
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 9

for both steel and copper are nearly two orders magnitude 

less than their convection coefficients, which implies 

the contribution from radiative heat loss is negligible. 

Experimental Methods

The study was designed to principally evaluate effects 

of bullet construction on ignition by bullet fragments 

themselves (not pieces of the target). Different calibers 

were used because these varied the velocity and bullet 

mass as well as constituent materials. Cartridges selected 

were based on the availability to the general public 

and the variety of bullet materials available (Table 3). 

Steel core ammunition was only widely obtainable for 

the 7.62x54R and the 5.56x45. Ammunition with steel 

jacketed bullets (referred to as Bi-metal) has thin copper 

gilding on the outside but is mostly made of soft steel. 

Copper jacketed and lead core bullets are the most common and come in many brands and varieties of jacket 

style. 

The target used for most tests was a steel bullet trap 

consisting of a deflector and a collector box (Figure 6a). 

The deflector was a 0.019 m (3/4 in) thick Abrasion 

Resistant (AR-500) steel plate 0.914 m (3 ft) wide by 

1.219 m (4 ft) long. The bottom edge of the deflector 

was connected by a hinge to the middle of the rim of a 

collector box with dimensions 0.9 m X 1.5 m X 0.3 m 

(3 ft X 5 ft X 1 ft) made of 0.006 m (1⁄4”) thick soft steel. 

The hinge allowed the deflector angle to be adjusted 

between 0 and 90 degrees from horizontal by means of 

Table 3—Rifles, cartridges, and bullets used for the study.

Rifle, Cartridge Manufacturer

Bullet 

weight 

(gr)

Muzzle 

velocity f s-1, 

(m s-1)

Muzzle 

energy 

ft-lbs, (J)

Internal 

construction

Jacket, 

Bullet style

Colt M4, 

5.56x451

Remington UMC 55 2900 (884) 2054 (2785) Lead Copper, FMJ

Lake City, M855 62 3000 (914) 2478 (3359) Hard Steel 

penetrator, lead

Copper, FMJ

Federal, Barnes 

TSX

55 2850 (869) 1984 (2689) Copper, solid none

Wolf WPA 55 2800 (853) 1915 (2596) Lead Steel FMJ 

Bimetal

Arsenal SA-M7, 

AK-47, 7.62x39

Barnaul, Silver Bear 123 2300 (701) 2889 (3917) Lead Steel, FMJ

Bimetal

Fiocchi 123 2450 (746) 3278 (4445) Lead Copper, FMJ

Mosin-Nagant, 

M91/30, 

7.62x54R

Hungary

(head stamp 21 74)

180 2600

(792)

2715 (3681) Steel, 

Soft

Steel, FMJ

Russia (188 head 

stamp, 1989)

149 2800 (853) 5187 (7032) Steel, Hardened Steel, FMJ

Barnaul, Silver Bear 174 2550 (777) 5024 (6811) Lead Steel, FMJ 

Bimetal

Winchester 180 2650 (808) 5613 (7610) Lead Copper, soft 

point

Springfield 

Armory M1A, 

7.62x512

Federal, American 

Eagle

150 2820 (860) 5296 (7181) Lead Copper, FMJ

Federal, Barnes 

TSX

150 2850 (869) 5410 (7335) Copper, solid none

Federal, Sierra 

SMK

168 2650 (808) 5238 (7102) Lead Copper, 

OTM

Nosler, Partition 165 2800 (853) 5744 (7787) Lead Copper, soft 

point

1 The designation 5.56x45 includes cartridges labeled as .223 Remington.

2 The designation 7.62x51 includes cartridges labeled as .308 Winchester.
10 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Figure 6—Images of the bullet trap (a) designed with an angle-adjustable deflector plate mounted in the middle of a steel collector 

box for bullet fragments deflected downward after impact from a horizontal trajectory. Pictures of the actual trap show (b) the 

secondary deflectors bolted to the outside edges of the deflector to redirect fragments to the center of the trap and (c) the collector 

box filled with peat for ignition tests. 

a)

b) c)

a cable and winch. Angle-iron was bolted to the edges 

of the deflector to redirect fragments into the collector 

(Figure 6b). For practicality, most tests were conducted 

by shooting directly at the steel deflector. A limited set 

of tests was conducted using granite slabs (3.2 cm [11⁄4 

in] thick) attached to the deflector plate. The purpose of 

using granite was to evaluate the possible role of nonsteel target materials in causing ignition. Granite was 

obtained as scrap from counter tops (approximately 30 

x 30 cm square) and oriented with the polished surface 

exposed to the bullet. The polished surface was painted 

with white spray paint to facilitate lighting with high 

speed videography. 

Shooting took place in the laboratory from a distance 

of about 32 m (35 yds). The bullet trap was housed inside 

a plywood shell to prevent fragments from damaging 

cameras or laboratory equipment (Figure 7). The following tests were conducted:

• Peat ignition tests on the steel target at combinations 

of cartridge, bullet type, and deflection angle. Five 

shots fired for each combination (Appendix).

• Peat ignition tests on granite target at 30 degree 

angle for selected combinations of bullet type and 

cartridge. Five shots fired for each combination.

• Excelsior ignition test (Barnes TSX only). Three 

shots fired.

• Bullet fragment temperature with empty collector 

box (IR camera).

• Bullet fragment temperature (“birdhouse” attachment to deflector plate, see Figure 9)

• Bullet fragment size distribution with deflector set 

to 30 degrees (water filled collector).
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 11

A chronograph (PACT Professional XP) was used 

to measure the muzzle velocity of each shot, with the 

averages for the test sequence reported in the Appendix. 

Most ignition tests were conducted with commercial peat 

moss that was oven dried at approximately 90 °C for 

two days and poured into the collector box to a depth of 

approximately 10 cm (4 in). Given the uncontrollability 

and uncertainty of bullet fragment properties (velocity, 

size, temperature, etc.), an ignition-sensitive material 

such as peat was thought necessary to allow ignition 

differences between bullet types to be distinguished. 

In other words, if ignitions were very rare, our limited 

set of tests may not be able to detect ignitions or make 

comparison among bullet materials. Peat was chosen 

because it is a partially decomposed organic substance 

similar to upper soil layers with large fractions of incorporated organic material. The ground surface would 

be a likely resting location for bullet fragments. Also, 

peat is composed of fine particles that would increase 

surface contact with small bullet fragments. 

Dryness of the peat became an important factor, as 

demonstrated on the 2nd day of indoor testing when the 

laboratory temperature was allowed to drop to about 

18 °C (65 °F) and relative humidity rose to the mid 

20-percent range. Peat moisture rose well above 6.5%. 

No ignitions were observed that day. Controls were put 

in place to maintain moisture conditions of the peat. In 

subsequent tests, dryness of the peat was maintained after 

removing it from the drying oven by heating the collector 

box underneath with heat tape and aiming halogen heat 

lamps along the sides. Measured temperatures of the box 

remained at approximately 55 °C, similar to soil surface 

temperatures on sunny summer days. The environment 

of the laboratory was sustained at temperatures of 38-

43 °C (100-110 °F) and approximately 7-10% relative 

humidity. This preserved moisture content of the peat 

between 3.0-4.5% (dry weight basis). Moisture sampling 

of the peat was performed approximately every 15 to 30 

minutes using a Computrac MAX 2000XL automatic 

balance. 

Ignitions were recorded after shooting each set of 

bullets by first observing the peat for smoldering spots 

(Figure 8). When observed, each spot was excavated 

with a small trowel. The volume of peat containing the 

smoldering spot was sifted on the pavement to attempt to 

find the fragment responsible. Once all visible ignitions 

had been removed, the trowels were used to thoroughly 

overturn the peat in the collector box. We noted that 

it often took several minutes before all ignitions were 

found – some being buried near the bottom of the peat. 

Once satisfied that no residual burning material was 

present, the next series of bullets was fired.

Fragment Temperatures

Measurement of bullet fragment temperatures was attempted by remote methods using a calibrated thermal 

IR video camera (Cincinnati TVS-8500) and directly 

by use of temperature sensitive paints. To capture IR 

images, the bullet collector box was emptied and the 

camera aimed to focus on the bottom surface with the 

deflector angle set to 30 degrees (from horizontal). IR 

video taken at 30 frames per second was analyzed by 

Figure 7—Bullet trap was housed in a plywood shell to shield laboratory equipment from stray fragments. Windows were cut in 

the doors for lighting and cameras.
12 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

tabulating maximum temperature of the pixels in the 

image over time to obtain cooling rates. The direct 

measurement of the approximate fragment temperature was attempted using a “birdhouse” attachment to 

the deflector/collector (Figure 9). The bullet was fired 

through the 7.6 cm hole (3 in) in the front plate. Fragments 

were contained inside the birdhouse and ricocheted off 

a series of baffles arranged to indirectly funnel them 

downward to rest upon a steel plate (0.159 cm [1/16 in] 

thick) coated with a temperature sensitive paint. Two 

tests were conducted at each paint temperature (300, 

400, 500, 600, 700, 800 °C) consisting of a single shot 

of 150gr 7.62x51 Barnes TSX. This bullet and cartridge 

was selected because ignitions consistently resulted 

during the peat tests and the fragments were not as 

damaging to the apparatus as steel-core bullets, which 

also readily produced ignitions. 

Statistical Analysis

We statistically examined the pairwise relationship 

between cartridge type and both ignition occurrence 

and number of ignitions via the Bonferroni method 

(Christiensen 1998, page 152) and the less conservative 

Tukey’s HSD test. In no case was there a statistically 

significant difference in the effect of cartridge types. 

We then examined the relationship between other predictors and ignition response via the use of Generalized 

Linear Mixed Modeling (GLMM) Poisson regression. 

The purpose of the regression analysis was to distinguish and characterize responses rather than to produce 

a predictive model since the data collected reflect the 

particulars of the laboratory testing such as number 

of shots and target distance. Although the number of 

separate ignitions in each set was recorded, the GLMM 

used only the binary responses of ignition or no-ignition. 

The dummy variables of bullet core material (lead, steel, 

copper) and jacket material (steel, copper) were specified 

in the model. 

Videography

Video in the visible portion of the spectrum was recorded for bullet impacts with a Photron Apex high 

speed video camera. Due to the plywood shell built 

around the bullet trap, and a desire to keep the high 

speed camera protected from fragments, camera and the 

lights were kept about 7m (20ft) away from the point of 

impact. Various recording rates were used, ranging from 

8,000 to 100,000 frames per second (fps), to attempt 

to capture impact fragmentation and impact flash. The 

various recording speeds were a result of the trade-off 

between resolution, aperture (for depth of field), and 

frame rate. When recording at 8,000 fps, resolutions 

as high as 1024x256 could be achieved, while recording at 100,000 fps resulted in a reduced resolution of 

only 128x32. It was determined that while the higher 

frame rates reduced the blur in the incoming bullet, the 

Figure 8—Photograph showing smoldering ignition from a hot bullet fragment as first detected in the peat. Smoldering spots are 

approximately 1 cm in diameter (0.4 inches).
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 13

fragmentation of the bullet after impact did not require 

nearly the same frame rates, as there did not appear to 

be any gain of additional information relative to the 

slower rate (8,000-24,000 fps). The slower frame rates 

also allowed for a stopping-down of the lens to increase 

the depth of field because the camera had to be placed 

oblique to the impact point (outside the travel path of the 

bullet but within full view of the impact point through 

the windows in the plywood shell). Attempts to capture 

real time contact between bullet fragments and the peat 

material failed after the first impact because thick dust 

obscured the view of the duff surface. Also a camera 

placed in front of the bullet lasted only one series before 

being destroyed by a fragment.

Results

A total of 469 rounds were fired for the tests, with 

433 against the steel target and 33 against granite (Appendix). The impact of rifle bullets against resistant 

targets consistently produced ignitions in dry peat. 

Ignitions were detected visibly as smoldering spots 

in the peat (Figure 8). Sometimes several separate 

ignitions were produced from the multiple fragments 

produced for a particular test (5 shots) (see Appendix). 

Sometimes several minutes went by before all ignitions 

were detected. This was interpreted as a function of the 

depth that a hot fragment was buried in the peat layer 

which required time for the ignition or smoke to become 

Figure 9—Photographs of the “birdhouse” attachment to the deflector plate on the bullet trap that allowed collecting and 

concentrating fragments onto plates painted with temperature sensitive paints: (a) Front view showing hole through which 

the bullet passes, (b) side view showing baffles and funnel, (c) top view showing location of plate in the bottom of the funnel.

a)

b) c)
14 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

visible on the surface. A single test for ignition of dry 

excelsior by the solid copper bullets (3 bullets) against 

the steel target produced ignitions. The tests using a 

granite target were inconclusive because of problems 

encountered with breakage by larger bullets and affixing the granite slabs tightly against the deflector for 

more than one shot. However, solid copper bullets did 

produce ignitions following impact with granite.

Fragments found by excavating ignitions in the peat 

suggested that bullet fragments were responsible for the 

ignition rather than steel eroded from the target. For steel 

jacketed bullets, very small fragments of jacket material were often found inside the incipient ignition. At 

low target angles, little cratering of the target occurred 

regardless of bullet type and limiting alternative sources 

of hot materials other than bullet fragments themselves. 

At high target angles using bullets with hardened steel 

penetrators, cratering of the steel target (Figure 10) could 

have liberated steel fragments and contributed to the 

ignitions. The process of deformation and fragmentation 

of the target would produce hot particles in the same 

way as discussed for bullet fragments. We did observe 

in one place where the sharp edge of a crater rim on the 

target had chipped from subsequent impacts and could 

have contributed hot material for ignition.

Figure 10—Cratering of AR-500 steel plate by steel core bullets 

was visible at high impact angle (60-80 degrees from horizontal) 

but not at low angles (20-40 degrees from horizontal).

Statistical analysis of the ignition results by Poisson 

regression revealed significant differences among bullet 

materials (Figure 11). The regression model (Table 4) 

represented separately the effects of core material and 

the jacket material compared to the base model that 

represented solid copper bullets (zeros for variables 

STEELCORE and LEADCORE and STEELJACKET). 

Bullet Material

Bullet construction materials were important factors in 

producing ignition (Table 1) (Figure 2). The only type 

of bullet that consistently did not produce ignitions 

was made with a lead core and copper jacket, although 

a single ignition was observed from a Nosler partition 

bullet. Two other ignitions resulting from lead core and 

copper jacketed bullets occurred immediately after 

shots involving solid copper bullets and were probably 

undetected hold-over ignitions from that test given their 

location in the collector that coincided with large areas 

of smoldering peat (Appendix). Solid copper bullets were 

the most consistent in producing ignitions at all angles 

and all targets. Fragments of the solid copper bullets 

appeared larger than fragments of other bullet types. 

Fragments recovered from the water-filled collector box 
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 15

Figure 11—Graph showing statistical regression model of laboratory data on 

probability of ignition as a function of bullet-type variables. Probability of ignition 

applies to 5-shot groups in oven dried peat (see Table 4).

Table 4—Regression estimates for Poisson model of ignition probability as a function of bullet core 

and jacket materials (see graph of regression functions in Figure 11). Core and jacket 

variables are set either to zero or one. If both are set to zero then regression produces 

results for solid copper bullets. All coefficients were statistically significant at least to the 

0.001 level (i.e., the probability of falsely finding non-zero coefficients).

Equation & coefficients Standard error z value Pr(> lzl)

IGNITION PROBABILITY = 1/(1+exp( -(2.57971 0.58514 4.409 1.04e-05

-3.90678 STEELCORE 0.66442 -5.880 4.10e-09

-2.35951 LEADCORE 0.65998 -3.575 0.00035

-1.66653 STEELJACKET 0.57410 2.903 0.00370

-0.03522 ANGLE))) 0.01209 -2.912 0.00360

supported this observation with solid copper bullets having the most combined weight of recovered fragments 

(Figure 12, Table 5). Bullets with lead core and copper 

jacket produced the smallest fragments with the least 

recovered weight. Bullets with steel components were 

found to produce ignitions but not as consistently as the 

solid copper bullets.

The regression model suggested that impact angle 

should also play a role in ignition probability, with 

more oblique angles more likely to produce an ignition. 

However, when the target was set at higher angles (60-

80 degrees) we suspected that more bullet fragments 

were escaping the collector box. The effect of angle on 

ignition would, therefore, involve more than effects on 

fragment properties (size or number). At high impact 

angles, fragments flying farther from the point of impact 

will experience more cooling before finally resting on 

potential ignitable substrate (Table 2) and maybe less 

likely to cause ignitions.
16 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Figure 12—Photographs of bullet fragments collected after impact from water-filled collector tank. Two rounds were fired for 

each test at the steel deflector set to 30 degrees from horizontal.

Table 5—Summary of bullet fragment tests using a water-filled collector box. Fragments were separated by material, weighed (all 

reported here in grains), and counted. See Figure 12 for photographs.

Cartridges	
  and	
  bullets	
  tested	
  for	
  

fragmentation

Weight	
  of	
  fragments	
  (grains) Number	
  of	
  

fragments

Total weight Core Jacket Core Jacket

Cartridge

Num.	
  

fired

Bullet	
  

wt	
  

(grains) Jacket Core

Weight

recovered

%

Original

weight

Max	
   Min Max Min

7.62x51 2 150 -­‐-­‐-­‐

Solid	
  

Copper 256.125 85.375 88.194 0.170 -­‐-­‐-­‐ -­‐-­‐-­‐ 41 -­‐-­‐-­‐

7.62x	
  51 2 150 Copper Lead 105.941 35.314 1.582 0.108 6.358 0.066 62 126

7.62x39 2 123 Steel Lead 104.938 42.658 1.173 0.123 12.099 0.093 50 54

7.62x54R 2 147 Steel

Steel/

Lead 199.443 67.838 27.500 0.154 9.275 0.011 49 58

a) 7.62x39, 123gr, Lead Core Steel Jacket C) 7.62x54R, 149gr, Steel Core Steel Jacket

b) 7.62x51, 150gr, Lead Core, Copper Jacket FMJ d) 7.62x51, 150gr, Solid Copper
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 17

Some of these ignitions were caught on the thermal 

camera (Figure 13), showing hot bullet fragments sitting on the peat and subsequently igniting. Interestingly, 

sometimes several minutes went by before all ignitions 

were detected. This was interpreted as a function of the 

depth that a hot fragment was buried in the peat layer, 

which required time for the ignition and smoke to become 

visible on the surface. For steel jacketed bullets, bullet 

fragments recovered from the ignition spots showed 

that shards of steel jacket were the dominant material 

associated with the ignitions. In addition to the thinness 

of the jacket, some of these fragments were surprisingly 

small, being only a few millimeters across. According 

to Hadden and others (2011), particles of this size require 

temperatures of 1100 °C or above to cause ignitions in 

dry cellulose. Whether the ignitability of the peat is 

greater than other materials is not known, but it should 

be investigated further.

High speed video (~20,000 frames per second) captured the impact and trajectory of splatter as well as an 

“impact flash” (Figure 14-Figure 20) that was visible for 

most bullets. Impact flash is not visible to the naked eye 

in daylight but is clearly visible in the videos. Bullets 

a) b)

Figure 13—Still-frames from thermal infrared camera showing (a) movement of bullet fragments 

after impact and (b) hot fragments resting on the floor of the bullet collector.

Figure 14—Frame sequence from high speed video (20,000 fps) of 147g 7.62x54R steel core full metal (steel) jacket bullet impacting 

steel plate at 20 degree angle (from horizontal) shows the “impact flash” produced by oxidation or burning of metal spall and hot 

glowing particles deflected downward.
18 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Figure 15—Frame sequence from high speed video (20,000 fps) of 147g 7.62x54R steel core full metal (steel) jacket bullet 

impacting steel plate at 30 degree angle (from horizontal) shows the “impact flash” produced by oxidation or burning of metal spall 

and hot glowing particles deflected downward.

Figure 16—Frame sequence from high speed video (20,000 fps) of 147g r 7.62x54R steel core full metal (steel) jacket bullet 

impacting steel plate at 40 degree angle (from horizontal) shows the “impact flash” produced by oxidation or burning of metal spall 

and hot glowing particles deflected downward.
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 19

Figure 17—Frame sequence from high speed video (20,000 fps) of 62gr 5.56x45 steel/lead core full metal (copper) jacket bullet 

impacting steel plate at 40 degree angle (from horizontal) shows the “impact flash” produced by oxidation or burning of metal spall 

and hot glowing particles deflected downward.

Figure 18—Frame sequence from high speed video (20,000 fps) of 174gr 7.62x54R lead core full metal (steel) jacket bullet 

impacting steel plate at 40 degree angle (from horizontal) shows a small impact flash and gray fragments.
20 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

Figure 19—Frame sequence from high speed video (20,000 fps) of 174gr 7.62x54R lead core full metal (steel) jacket bullet 

impacting steel plate at 60 degree angle (from horizontal) shows a small impact flash and gray fragments.

Figure 20—Frame sequence from high speed video (20,000 fps) of 150gr 7.62x51 solid copper bullet impacting steel plate at 60 

degree angle (from horizontal) shows a small “impact flash” and large fragment.
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 21

having a steel core displayed the greatest and longestlasting flash (Figure 14-Figure 17). Smaller flash was 

visible from impact of bullets with lead core (Figure 18 

& Figure 19) and of solid copper (Figure 20). Impact 

flash has been described as the burning of metal spall 

(oxidizing metal dust from the target and/or projectile) 

at temperatures of about 2700 °C (5000 °F) (Abernathy 

1968, Mansur 1974). Its duration is very short and was 

visible in high speed video for less than 1/2000th of a 

second (<10 frames at ~20000 frames per second).

Temperatures of Bullet Fragments

The thermal camera captured 30 frames per second 

and recorded the movements and approximate temperatures of the fragments as they bounced around in the 

collector (Figure 21a). Assuming fragment emissivity 

to be 1.0, the thermal images recorded peak temperatures of 550-793 °C. We attribute little significance to 

the apparent variability among bullets or shots because 

there was no control over fragment numbers, sizes, or 

locations in the field of view. Also, for several reasons 

discussed below, these estimated temperatures must be 

considered conservative values that may be affected 

by factors beyond the control of the experiments. Even 

without confidence in the actual fragment temperature, 

these data are indicative of high thermal energy of the 

particles and the rapid cooling rates following impact. 

Higher cooling rates of the steel-components were 

indicated (Figure 21b, c) compared to fragments of the 

solid copper bullets (Figure 21d). The steel components 

cooled within several seconds to temperatures near the 

Figure 21—A thermal camera operating at 30 fps captured images of bullet fragments on the bottom of the collector box 

after impact (A). A time-sequence of maximum temperature in each frame shows recorded bullet fragment temperatures 

(B, C, D) for different bullet types. 
22 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

minimum setting for the camera (375 °C) but copper took 

approximately three times longer. This may be caused 

by higher initial temperatures or partially a function of 

the larger sizes of the copper bullet fragments because 

more rapid cooling of copper (for equivalent mass) 

would be expected from the higher thermal conductivity 

compared to steel (almost 10 times greater).

Examination of the temperature-sensitive plates 

from the “birdhouse” tests revealed discoloration from 

contact points with hot fragments at all temperature 

levels, including the maximum of 800 °C (Figure 22 

and Figure 23). The density of discolored places decreased with increasing temperature threshold for the 

paints (Figure 22 and Figure 23), suggesting that most 

particles cool too rapidly to discolor the paint or are not 

raised to higher temperatures initially. Given the single 

temperature threshold of each paint and the thermal 

conductivity and limited contact on flat steel plates, 

these results are necessarily conservative estimates of 

actual fragment temperature. They are consistent with 

both the theory and the thermal camera data, which 

reveal fragment temperatures sufficient for ignition.

Discussion

No previous studies have been devoted to the particular 

problem of ignition by metal fragments heated upon ballistic impact. The physical processes and factors involved 

throughout the sequence of impact, fragmentation, and 

ignition are, nevertheless, interpretable from studies of 

related phenomena. In general, high velocity impacts 

produce heat in the rapidly deformed projectile (and 

Figure 22—Plate with paint sensitive to 300 °C. Paint turns from brown to dark brown at 300 °C. For higher 

temperatures, the paint turns yellow, and for sufficiently high temperatures, black. Top left: Plate with fragments 

from one Barnes solid copper TSX bullet as removed from the birdhouse collector. Top right: Plate cleared of 

debris. Bottom left: Close up views of portions of the plate highlighting some of the color changes. Bottom right: 

Bullet fragments ranging in size from less than 1 mm to 2 cm.

A

A

B

B

C

C

300 °C
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 23

possibly the target) that must quickly come to rest on a 

dry ignitable material. The rapid cooling of fragments, 

their small sizes, and odd shapes mean that fine-grained 

substrates such as peat provide more opportunity for 

direct contact and ignition. The toughness of different 

metals determines how much energy is required for 

deformation and fracturing, and thus, how much heat 

is generated when the particle material ultimately fails. 

This is why bullet construction is important to heating 

and ignition.

Our study focused on laboratory testing of different 

bullets loaded in commercial ammunition. We observed 

that bullet material did affect fragment sizes and ignitions, with steel components and solid copper bullets 

producing the largest fragments and the most likely 

ignitions in peat. Despite similar maximum temperatures 

recorded on thermal images, larger fragments from solid 

copper bullets seem to be the most plausible explanation 

for the slower rates of temperature decline compared to 

steel seen in the image sequences (Figure 21). The opposite trend would be expected based only on the greater 

thermal conductivity of copper than steel, meaning that 

heat loss rates should be greater for equivalent fragment 

mass. Both copper and steel are much “tougher” metals 

compared to lead, meaning that a greater amount of 

energy is required for plastic deformation at a particular 

strain rate. Lead therefore deforms with relatively little 

energy, and due to its relatively low melting temperature, 

will probably melt. 

The actual bullet fragment temperatures remain unknown, but they are consistent with the physical theory 

of plastic deformation under high strain rates. Maximum 

temperatures of about 550 °C to nearly 800 °C were 

recorded on thermal images for several bullet types 

Figure 23—Steel plate with paint sensitive to 800 °C. Paint turns from blue to dark blue at 800 °C. For 

higher temperatures, the paint turns pink, and for sufficiently high temperatures, brown. Top left: Plate with 

fragments from one Barnes solid copper TSX bullet as removed from the birdhouse collector. Top right: 

Plate cleared of debris. Bottom left: Close up views of portions of the plate highlighting some of the color 

changes. Bottom right: Bullet fragments ranging in size from less than 1 mm to 2.5 cm.

800 °C

A

C

B

A

C

B
24 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

and were consistent with the discoloration reaction by 

fragment contact with the temperature-sensitive paints. 

Both must be considered conservative estimates of the 

true fragment temperatures because:

1. The thermal camera was operated at 30 fps and 

could miss high temperatures of shorter duration. 

2. There is an unknown and uncontrolled ratio of 

bullet fragment size relative to the pixel area in 

an image, which can lead to under-representing 

temperature due to partial pixel coverage by the 

fragment. 

3. The emissivity of the bullet fragments is not known 

but assumed to be 1.0 for these calculations and 

which must therefore underrepresent the actual 

temperature.

4. Irregular particle geometry offers few contact points 

on a flat steel plate for discoloring the paints.

5. Thermal properties of the steel plate may diminish 

the paint response to small fragments.

This study intentionally did not address ignition by 

target material dislodged by the impact. Most ballistic 

impact studies are concerned with perforation or penetration of the target, often metal, which may break away. 

Loose pieces of target metal can be fractured by similar 

deformation physics as described for the projectile, and 

it is possible that fires could ignite from them as well.

From this study, an understanding of wildfire ignitions 

from field reports begins to emerge, but also involves 

other processes not encompassed by this work. The following is a discussion of linkages to field-scale wildfire 

ignitions, given that the present study was confined 

to a laboratory apparatus and limited to detection of 

smoldering ignition very close to the target (<1m):

• Target materials that are highly resistant to damage would be similar to the steel plates and granite 

slabs tested here, such as boulders, rocks, or thick 

metal such as silhouettes. Oblique angles of impact 

may be important, regardless of target material, to 

producing larger fragments that would cool more 

slowly after contacting organic matter. 

• Bullet materials clearly affect ignition potential, 

with steel components and solid copper having the 

greatest chance of producing hot fragments. We 

observed only one ignition from lead-core copper 

jacketed bullets.

• The very rapid particle cooling means the ignitions 

are more likely nearer the target. Fragment size distribution was not known or controlled, but smaller 

pieces cool so quickly that they must contact the 

suitable substrate very rapidly. The distances from 

a target that ignitions can occur are not determined 

by the present study.

• Ignitions are observed in the field only when the 

fire begins to spread. This is probably not when or 

where ignition actually takes place. The original 

ignition likely occurs in a material similar to peat, 

meaning partially decomposed organic matter incorporated in the surface horizons of the soil – not 

the vegetation or fuel which carries the spreading 

fire with visible flames. The process of transition 

from smoldering incipient ignition to spreading fire 

may take some time (minutes to days, even weeks) 

depending on the fuel types and the weather and 

fuel conditions. Where the target is exposed to 

wind, a smoldering ignition in litter or duff may 

be ventilated easily and ignite grasses or surface 

litter and become visible more quickly than an area 

sheltered by trees or terrain. 

• Consistent with previous research on particle ignition, the finding that dry excelsior could be ignited 

by the large pieces of solid copper bullets suggests 

that larger particles can start fires more easily, but 

small particles may require fine-grained or rotten 

material. This would require further testing beyond 

this study to determine however.

• Bullet fragments can be very small and still effective in producing ignitions. The multiple ignitions 

observed in this experiment from small fragments 

of a single bullet means that it may be difficult to 

identify the exact piece of bullet material that causes 

an ignition under field conditions.

• As with all fire behavior and ignition research, 

moisture content of the organic material will be an 

important factor in ignition. Peat moisture contents 

of 3-5%, air temperatures of 34-49 °C (98-120 °F), 

and relative humidity of 7 to 16% were necessary 

to reliably observe ignitions in the experiments. 

Peat moisture contents above this (perhaps 8%) did 

not produce ignitions. Field conditions matching 

the experimental range would imply summer-time 

temperatures, as well as solar heating of the ground 

surface and organic matter to produce a drier and 

warmer microclimate where bullet fragments are 

deposited. 
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 25

Acknowledgments

Conducting this study depended on the support and 

ingenuity of many people. Chuck Harding constructed 

the bullet trap. Jack Kautz, Randy Pryhorocki, and 

Andrew Gorris built and designed the laboratory shooting 

range apparatus including external sound suppressor, 

birdhouse attachment to the deflector, and granite 

attachment assembly. Mark Vosburgh, Amanda Determan, and Dan Jimenez took responsibility for video 

imagery. Jay Fronden and Jason Forthofer assisted 

in all phases of the shooting tests. Mike Bonzano 

ensured range safety for the laboratory tests. Dave 

Ball of the Missoula County Sheriff’s Department 

graciously helped with the access and use of the outdoor range. Isaac Grenfell provided data processing 

and statistical advice and analysis. Shari Kappel and 

Corrie Kegel oversaw safety and security plans for 

the indoor testing. This report was greatly improved 

from the review comments provided by Don Latham, 

Vytensis Babrauskas, Steve Mates, and John Fehr.
26 USDA Forest Service Res. Pap. RMRS-RP-104. 2013

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USDA Forest Service Res. Pap. RMRS-RP-104. 2013 27 Appendix—Data obtained from shooting tests. Red shading indicates tests where ignitions were observed. Yellow shading indicates ambiguous result (see comments) Date Test number Target material Cartridge Manufacturer Bullet type Angle Material Number of rounds Number of ignitions Avg muzzle velocity Comments Weight Core Jacket Deg ft s-1 Outdoor Tests 9/21/2012 x8 steel 5.56x45 m855 62gr Steel/ Lead Copper 30 Peat 11 0 9/21/2012 x9 steel 7.62x54R Hungarian Surplus 180gr FMJ Steel/ Lead Steel 30 Peat 5 2 9/21/2012 x10 steel 7.62x54R Hungarian Surplus 180gr FMJ Steel/ Lead Steel 20 Peat 5 5 9/21/2012 x11 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 20 Peat 7 7 Rifle: Russian Mosin-Nagant 91/30 1/22/2013 1 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 20 Peat 5 4 2813 1/22/2013 2 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 30 Peat 5 2 2827 1/22/2013 3 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 40 Peat 5 2 2813 1/22/2013 4 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 60 Peat 5 4 2776 1/22/2013 5 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 80 Peat 5 1 2806 1/22/2013 6 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 20 Peat 5 4 2595 1/22/2013 7 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 30 Peat 5 0 1/22/2013 8 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 40 Peat 5 0 1/22/2013 9 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 60 Peat 5 0 2592 1/22/2013 10 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 80 Peat 5 0 2603 1/23/2013 11 steel 7.62x54R Winchester 180gr JSP Lead Copper 20 Peat 5 0 2641 1/23/2013 12 steel 7.62x54R Winchester 180gr JSP Lead Copper 30 Peat 5 0 2657 1/23/2013 13 steel 7.62x54R Winchester 180gr JSP Lead Copper 40 Peat 5 0 2639 1/23/2013 14 steel 7.62x54R Winchester 180gr JSP Lead Copper 60 Peat 5 0 2647 1/23/2013 15 steel 7.62x54R Winchester 180gr JSP Lead Copper 80 Peat 5 0 2634 Rifle: AK47, Arsenal SA-M7A1R 1/23/2013 16 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 20 Peat 5 0 2325
28 USDA Forest Service Res. Pap. RMRS-RP-104. 2013 1/23/2013 17 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 30 Peat 5 0 2312 1/23/2013 18 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 40 Peat 5 0 2320 1/23/2013 19 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 60 Peat 5 1 2331 1/23/2013 20 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 80 Peat 5 0 2324 1/24/2013 21 steel 7.62x39 Fiocchi 123gr FMJ Lead Copper 20 Peat 5 0 2442 1/24/2013 22 steel 7.62x39 Fiocchi 123gr FMJ Lead Copper 30 Peat 5 0 2470 1/24/2013 23 steel 7.62x39 Fiocchi 123gr FMJ Lead Copper 40 Peat 5 0 2452 1/24/2013 24 steel 7.62x39 Fiocchi 123gr FMJ Lead Copper 60 Peat 5 0 2443 1/24/2013 25 steel 7.62x39 Fiocchi 123gr FMJ Lead Copper 80 Peat 5 0 2462 Rifle: AR15, Colt M4 LE6940 1/24/2013 26 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 20 Peat 5 1 2996 1/24/2013 27 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 30 Peat 5 0 2997 1/24/2013 28 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 40 Peat 5 1 2984 1/24/2013 29 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 60 Peat 5 0 2997 1/24/2013 30 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 80 Peat 5 0 3022 1/24/2013 31 steel 5.56x45 Wolf 55gr FMJ Lead Steel 20 Peat 5 0 2884 1/24/2013 32 steel 5.56x45 Wolf 55gr FMJ Lead Steel 30 Peat 5 0 2884 1/24/2013 33 steel 5.56x45 Wolf 55gr FMJ Lead Steel 40 Peat 5 0 2863 1/24/2013 34 steel 5.56x45 Wolf 55gr FMJ Lead Steel 60 Peat 5 0 2839 1/24/2013 35 steel 5.56x45 Wolf 55gr FMJ Lead Steel 80 Peat 5 0 2868 1/25/2013 36 steel 5.56x45 Remington 55gr FMJ Lead Copper 20 Peat 5 0 2942 1/25/2013 37 steel 5.56x45 Remington 55gr FMJ Lead Copper 30 Peat 5 0 2924 1/25/2013 38 steel 5.56x45 Remington 55gr FMJ Lead Copper 40 Peat 5 0 2924 1/25/2013 39 steel 5.56x45 Remington 55gr FMJ Lead Copper 60 Peat 5 0 2969 1/25/2013 41 steel 5.56x45 Federal 55gr Barnes TSX Copper Copper 20 Peat 5 2 3001
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 29 1/25/2013 42 steel 5.56x45 Federal 55gr Barnes TSX Copper Copper 30 Peat 5 1 3027 1/25/2013 44 steel 5.56x45 Federal 55gr Barnes TSX Copper Copper 60 Peat 5 0 3052 Rifle: M1A, Springfield Armory 1/25/2013 51 steel 7.62x51 Federal 150gr FMJ Lead Copper 20 Peat 5 1 2888 Probably hold- over ignition from test 44 1/25/2013 52 steel 7.62x51 Federal 150gr FMJ Lead Copper 30 Peat 5 0 2883 1/25/2013 53 steel 7.62x51 Federal 150gr FMJ Lead Copper 40 Peat 5 0 2796 1/25/2013 54 steel 7.62x51 Federal 150gr FMJ Lead Copper 60 Peat 5 0 2835 1/25/2013 57 steel 7.62x51 Federal 150gr Barnes TSX Copper Copper 30 Peat 5 12 2913 1/25/2013 58 steel 7.62x51 Federal 150gr Barnes TSX Copper Copper 40 Peat 5 7 2903 1/25/2013 59 steel 7.62x51 Federal 150gr Barnes TSX Copper Copper 60 Peat 5 6 2831 1/25/2013 60 steel 7.62x51 Federal 150gr Barnes TSX Copper Copper 80 Peat 5 0 2832 0 1/25/2013 61 steel 7.62x51 Federal 168gr SMK OTM Lead Copper 20 Peat 5 0 2649 1/25/2013 62 steel 7.62x51 Federal 168gr SMK OTM Lead Copper 30 Peat 5 1 2645 Probable hold- over ignition from test 60 1/25/2013 63 steel 7.62x51 Federal 168gr SMK OTM Lead Copper 40 Peat 5 0 2634 1/25/2013 66 steel 7.62x51 Nosler 165gr Partition Lead Copper 20 Peat 5 1 2875 1/25/2013 67 steel 7.62x51 Nosler 165gr Partition Lead Copper 30 Peat 5 0 2860 1/25/2013 68 steel 7.62x51 Nosler 165gr Partition Lead Copper 40 Peat 5 0 2786 1/23/2013 76 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 40 Peat 5 0 2834
30 USDA Forest Service Res. Pap. RMRS-RP-104. 2013 1/23/2013 77 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 40 Peat 5 0 2827 1/23/2013 78 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 40 Peat 10 0 2319 1/24/2013 79 steel 7.62x54R Russian Surplus 147gr FMJ Steel Steel 30 Peat 5 6 2831 1/24/2013 80 steel 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 40 Peat 10 0 1/24/2013 6 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 20 Peat 5 1 2620 1/24/2013 7 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 30 Peat 5 0 2621 1/24/2013 8 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 40 Peat 5 2 2631 1/24/2013 16 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 20 Peat 5 4 2326 1/24/2013 17 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 30 Peat 5 0 2339 1/24/2013 18 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 40 Peat 5 1 2355 1/24/2013 19 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 60 Peat 5 0 1/24/2013 20 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 80 Peat 5 0 1/25/2013 8 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 40 Peat 5 0 1/26/2013 6 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 20 Peat 5 2 2677 1/26/2013 7 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 30 Peat 5 1 2636 1/26/2013 8 steel 7.62x54R Silver Bear 174gr FMJ Lead Steel 40 Peat 5 1 2623 1/26/2013 16 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 20 Peat 5 1 2347 1/26/2013 17 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 30 Peat 5 0 2330 1/26/2013 18 steel 7.62x39 Silver Bear 123gr FMJ Lead Steel 40 Peat 5 0 2334 1/26/2013 31 steel 5.56x45 Wolf 55gr FMJ Lead Steel 20 Peat 5 0 2898 1/26/2013 32 steel 5.56x45 Wolf 55gr FMJ Lead Steel 30 Peat 5 2 1/26/2013 33 steel 5.56x45 Wolf 55gr FMJ Lead Steel 40 Peat 5 0 2864 1/26/2013 2 granite 7.62x54R Russian Surplus 147gr FMJ Steel Steel 30 Peat 2 0 2858 granite broke 1/26/2013 7 granite 7.62x54R Silver Bear 174gr FMJ Lead Steel 30 Peat 4 0 2655 granite broke 1/26/2013 17 granite 7.62x39 Silver Bear 123gr FMJ Lead Steel 30 Peat 5 0 2353 1/26/2013 17 granite 7.62x39 Silver Bear 123gr FMJ Lead Steel 30 Peat 5 0 2345 granite broke
USDA Forest Service Res. Pap. RMRS-RP-104. 2013 31 1/26/2013 27 granite 5.56x45 Lake City 62gr FMJ Steel/ Lead Copper 30 Peat 5 0 3041 1/26/2013 32 granite 5.56x45 Wolf 55gr FMJ Lead Steel 30 Peat 5 0 2917 1/26/2013 42 granite 5.56x45 Federal 55gr Barnes TSX Copper Copper 30 Peat 5 9 3061 1/26/2013 57 granite 7.62x51 Federal 150gr Barnes TSX Copper Copper 30 Peat 2 1 2905 granite broke 1/26/2013 57 steel 7.62x51 Federal 150gr Barnes TSX Copper Copper 30 Excelsior 3 3 2906
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