THREAT CHARACTERIZATION: SMALL PROJECTILES, BIG IMPACT
By: Jonathan Marshall
The global proliferation of assault rifles, machine guns, and other man-portable weapons has increased both the frequency and intensity of modern conflict. These weapons, classified as small arms (which typically include up to 23 mm in caliber), are easily obtainable on international markets and are effectively used by untrained personnel. There are an estimated 100 million to 500 million military-style weapons in circulation, in addition to hundreds of millions designed for police or civilian use. In addition, of the 49 major conflicts that have occurred since 1990, small arms and light weapons (such as those shown in Figure 1) were the primary weapons used in 46 of them. Only the Gulf War predominately used heavy weapons [1, 2].
Several distinguishing features of small arms make them suitable for modern conflicts. First, their previously mentioned wide proliferation makes them easily obtainable. This availability is attributed to new production, remnants of downsizing militaries, or recycled weapons from previous conflicts. Additionally, the rapid-fire capability of small arms (often of 300+ rounds per minute) allows a single individual to pose a significant threat to military systems. The simplicity and low maintenance of small arms also greatly increase their use in conflicts involving untrained combatants, as these weapons require little training to use effectively as well as little logistical support. Finally, small arms and light weapons can be carried by an individual soldier or light vehicle and are easily transported or smuggled to areas of conflict. Not surprisingly, these combined characteristics of small arms have made them particularly attractive to the paramilitary and irregular forces that have played such a prominent role in recent conflicts.
In addition to the significant threat they pose to personnel, small arms projectiles are designed to penetrate airframes and disable flight-critical systems. Armor piercing-incendiary (API) projectiles are designed to release incendiary material into a dry bay when the projectile jacket is stripped. This incendiary material can burn and linger in the dry bay long enough to interact with fuel and initiate fires, which can be a large contributor to aircraft vulnerability.
Small arms threats also have limited countermeasures as they are point-to-point munitions guided by optics that have no sophisticated acquisition or guidance systems. The primary countermeasures are concept of operations (CONOPS) and tactics, training, and procedures (TTPs). For example, low-visibility conditions, (e.g., night operations) help to conceal an aircraft from engagement. Operators can also avoid engagement by flying outside of the effective range of small arms weapons. However, many military platforms are often used for low-altitude operations within the threat engagement range (e.g., helicopters, such as the Russian one shot down by ground fire in Figure 2, often fly “low and slow” during certain mission points).
If countermeasures fail or are unavailable, vulnerability enhancement features can be used to minimize the risk of small arms. Examples of vulnerability enhancements include increasing redundant systems or adding armor; however, both actions increase cost and impact performance metrics.
Aircraft vulnerability and the benefits of vulnerability enhancement features are measured and quantified using modeling and simulation (M&S) and test & evaluation (T&E). The purpose of M&S and T&E is to develop a specific understanding of component-, system-, and aircraft-level vulnerabilities. A detailed vulnerability analysis data set is developed and combined with a representative three-dimensional geometric model to conduct computer-based simulations to quantify vulnerabilities. Simulations and testing can assess the impact of vulnerability reduction features and are often used for specification compliance.
THE IMPORTANCE OF THREAT CHARACTERIZATION
Threat characterization has an important role in vulnerability analyses. Damage inflicted to components in an analysis is measured using specific threat parameters, such as projectile physical characteristics, incendiary mass, and incendiary functioning potential. Every threat used for a vulnerability analysis must be fully characterized to properly assign lethality and probabilities of damage or kill. Over the years, M&S tools and models have improved in fidelity and accuracy. As these analytical tools and techniques evolve and improve, our understanding of threats must follow suit. The Director, Operational Test & Evaluation (DOT&E) recognized this need and funded efforts to fully characterize and evaluate the most significant dry bay fire parameters. Also, ProjPen and FATEPEN developers continuously investigate and field advancements in threat simulation tools.
Ballistic testing and data collection techniques have also improved in recent years. For example, an increase in the depth and use of high-speed video cameras has allowed analysts to decipher minute differences in the effects of small arms projectiles. Recent efforts have identified that these differences have statistical significance. It is imperative to understand the differences between threats, as these data are repeatedly used for verification against program specification requirements. Accurate threat data must be captured to perform an accurate vulnerability analysis. These threat data include physical characteristics and incendiary function data for API projectiles. It is critical to ensure the physical properties and functioning of a projectile are understood, as these factors can significantly alter expected T&E results. Correlation between T&E and M&S can also be difficult to track and understand if different projectiles are unknowingly being used.
Threat physical characteristics include projectile dimensions and material data. Projectile dimensions such as length, diameter, and core length and diameter are important to accurately measure across projectile lot samples to identify variations. These variations primarily stem from the wide proliferation across multiple countries, the number and quality of manufacturers, and the circulation of multiple threat types and designs, of which many pre-date the Cold War era. Variations in projectiles can be hidden from exterior appearance (as noted by the variations in identical caliber threats illustrated in Figure 3).
Differences in core material, hardness, and tensile strength are important to accurately sample. Projectile dimensions and material data are crucial to precisely capture penetration capabilities and characteristics. For example, length and diameter affect hole size, whereas core hardness and tensile strength affect penetration and threat break-up. Specific to APIs, function data include incendiary mass and location within a projectile as well as function cloud size and duration. The amount of incendiary within a projectile determines maximum function cloud size and duration and can vary greatly between threats, which can cause a large variance in function cloud size and duration. Small-caliber threats may have an incendiary mass on the order of 10 grains, whereas a large-caliber threat may have an incendiary mass of 70 grains or more. In addition, the location of the incendiary may determine a threat’s function occurrence rate. Differences in incendiary mass location can affect how the incendiary material functions as the projectile jacket is stripped away.
Several characterization methods are used to measure the projectile characteristics that play a significant role in survivability. Computer tomography (CT) scanning, physical exploitation, and ballistic characterization testing are often used in conjunction to build a thorough data set for a specific projectile. These data sets detail variations or define the physical properties of a specific round.
CT scanning combines a series of X-ray images taken from different angles to produce cross-sectional images. Computer processing is used to generate a three-dimensional image of the inside of the projectile from two-dimensional radiographic images taken about a single axis of rotation (as shown, for example, in the variation in the lead jackets pictured in Figure 4). CT scanning is nondestructive evaluation (NDE) and allows a detailed view of the inside of a projectile. The scans can identify the density of objects constructed of metal, ceramic, and composite and can provide accurate measurements to build three-dimensional geometric models. CT scanning is cost-effective and can scan multiple projectiles at the same time.
Physical exploitation, or threat dissection, is the process of creating cross sections of a projectile by mounting samples in epoxy and then cutting or grinding and polishing the mount for imaging and testing. Threat dissection is destructive in nature; however, it is the only means to physically sample the interior of a projectile. Material testing, such as core hardness, can only be conducted on an exposed surface. Additionally, incendiary masses can be examined for specific compositions.
Ballistic characterization testing is used to analyze penetration, projectile dynamics, and function characteristics. Typical characterization testing fires projectiles from a sample lot into mounted panels (pictured in Figure 5). Several input factors are varied to accurately capture variations in projectile dynamics. These input factors include initial velocity, initial yaw, panel obliquity, panel material, material thickness, and the airgap between panels (if testing multiple panels). High-speed video cameras are used in ballistic testing to record function type, cloud size, duration, and mobility. These high-speed cameras are capable of image exposures in microseconds. The images are then played in slow motion to determine function type (front face function and back face function), cloud size, and duration.
CURRENT AND FUTURE EFFORTS
Currently, the Air Force Life Cycle Management Center’s Combat Effectiveness and Vulnerability Analysis Branch (AFLCMC/EZJA) maintains Pedigree documents that provide the analytical foundation for weapon system survivability and effectiveness analyses. The “Pedigree Document – Guns”  report provides threat-based input files for vulnerability models used throughout the community. The data were primarily derived from intelligence sources and were reformatted to be compatible with the vulnerability models. The Pedigree documents detail several commonly used threats for M&S vulnerability analyses. As new data (e.g., new physical exploitation or CT scans) are received, the data are reviewed and summarily updated within the Pedigree documents.
Recently, several threats have had function characterization testing completed. These data were used to statistically analyze each threat to determine the probabilities of each function type. The Pedigree documents are currently one of the best sources to identify and characterize a threat to be used for analyses.
Looking to the near future, threat characterization will play a large role in upcoming prediction models. The Joint Aircraft Survivability Program (JASP) has funded the Next Generation Fire Model (NGFM), which aims to improve fire prediction capability by improving penetration, energy deposition, fuel deposition, and ignition. The threat data quality is of primary importance to enhancing the predictive capability of these models.
Detailed threat characterization begins with the development of acquisition program survivability requirements. With so many countries and manufacturers of small arms projectiles, there are several variations for threats of the same caliber. Although recent programs have been more specific in the threat designation, it is imperative to be as specific as possible when defining projectiles to be evaluated (i.e., 7.62×39mm BZ M1943 API), as these specifics will drive the M&S and T&E requirements. Even minor variations in projectile designation can influence vulnerability analyses results, which can be enough to alter specification requirement compliance.
It is also recommended that any future ballistic test program conduct a full threat characterization to quantify the important parameters that can influence vulnerability results. Whenever a new lot of projectiles is procured, a physical exploitation should be conducted to characterize the size and materials of the projectile. In addition, CT scanning should be conducted to characterize volumes and identify anomalies not easily identified in physical exploitation for use in vulnerability analyses. This projectile characterization information should then be compared back to existing threat data to ensure that the size and compositions are similar. If there are variations, then ballistic characterization testing should be completed to evaluate the penetration and/or functioning potential of the projectile. These data would then be used to update and enhance the vulnerability M&S tools. Small proactive investments to understand and characterize threats prior to a significant LFT&E series can prevent ambiguity due to variations and can ensure proper integration of results for verification of key performance parameters and specification compliance.
ABOUT THE AUTHOR
Mr. Jonathan Marshall is a vulnerability analyst at the SURVICE Engineering Company. For the past 4 years, he has supported aircraft survivability testing and modeling for multiple rotorcraft and fixed-wing programs. Mr. Marshall holds a B.S. degree in mechanical engineering from Wright State University.
References  Boutwell, Jeffrey, and Michael Klare. “Small Arms and Light Weapons: Controlling the Real Instruments of War.” Arms Control Association, https://www.armscontrol.org/act/1998_08-09/ mkas98, 1 August 1998.  Federation of American Scientists. “The Global Threat of Small Arms and Light Weapons—A Primer.” https://fas.org/asmp/campaigns/smallarms/ primer.html, accessed June 2017.  Air Force Life Cycle Management Center and the SURVICE Engineering Company, “Pedigree Document – Guns: Non-Exploding Vulnerability Threat Data.” Wright-Patterson AFB, OH, June 2017.