Morphological study of fly-ash block under angular impact of 9 mm projectile

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BACKGROUND

Forensic investigators use different methods to establish the forensic significance of fired projectiles. These methods help them in conducting effective investigations to support legal proceedings. When a projectile hits a target, the outcomes are contingent on the encountered barriers. The effects of one or more interactions between a projectile and barriers that prohibit its penetration at potentially lethal velocities for vulnerable components constitute the terminal ballistics of a projectile. The primary goal of a barrier is to decelerate the projectile, achieved through fragmentation or deformation. The projectile’s velocity is partially influenced by factors including propellant type, ignition, and subsequent high-pressure gas expansion that propels the projectile from the weapon’s barrel at a substantial muzzle velocity [1, 2]. Understanding the mechanical behavior of solids is essential to establish a foundation for selecting barrier parameters crucial in defining terminal interactions [3]. Concrete, showing distinct characteristics under tension and compression, presents a more intricate challenge in ballistic assessment compared to metals. These characteristics help in identifying the firearm used and potential firing locations. Significant differences in the physical properties of metals, concrete, and masonry govern the resisting forces against penetration and the fracture mechanisms leading to perforation. Concrete structures under firearm attacks can experience diverse fracture modes, including complete fracture, penetration, perforation (full penetration), scabbing, spalling, cracking, local plugging, and/or global failure (as shown in Figure 1) [4]. Among these, perforation inflicts the most severe damage, while spalling and scabbing, accompanied by craters at entry and exit points, represent common failure modes [2].

 

Fig. 1. Local Damages and Global Failure.

 

The trajectory of a bullet holds crucial importance in reconstructing crime scenes, significantly impacting bullet performance. As a bullet exists the barrel, it follows a parabolic trajectory, curving downward. Regardless of the muzzle velocity, the bullet experiences an approximate 1.15 m drop in half a second of flight. Point-blank range shots exhibit an almost flat trajectory. The angle of elevation, muzzle velocity, and bullet shape collectively determine the bullet’s maximum attainable range. Reconstructing a crime scene based on bullet trajectory involves analyzing primary bullet defects, shape, dimensions, and the spatial relation among primary, secondary, and tertiary bullet defects [1, 5].

AIM

The reconstruction of firearm-involved crime scenes requires identifying fired-shot characteristics. This study aims to determine bullet trajectories, aiding in establishing the firing direction, location, suspect height, and firing angle. Through meticulous analysis of damages caused by a 9mm full metal jacketed projectile on AAC blocks at various angles, diverse possibilities were obtained. Therefore, this study spans different firing ranges for angles of 0°, 15°, 30°, and 45°. The objectives are to determine the shooter positions, establish a connection between energy loss and the damage caused by the 9 mm projectile on the AAC block with varying firing angles, and establish a relationship between the bullet trajectory and angles by considering the structural dimensions of entry and exit points.

MATERIALS AND METHODS

Sample Preparation and Projectile Selection

The study utilized AAC blocks manufactured in accordance with BIS Standard for Concrete Masonry Units Part 3 Autoclaved Cellular (Aerated) Concrete Blocks (IS 2185 Part-3:1984). AAC blocks are made by the formation of calcium silicate hydrate, achieved by reacting sand with calcium hydroxide under steam and pressure curing in an autoclave for 14 to 18 hours. These blocks, commonly employed in constructing walls for houses and bridges, were suitable for the study’s objectives. The physical specifications of the AAC blocks are detailed in Table 1 [6, 7].

 

Table 1. Physical Specifications of AAC Block

Test Parameters (Physical Tests)	Specified Requirements As Per IS 2185 (Pt-3):1984	Observed Value	Test Method
Length Height Width	600±5 mm 200±5 mm 230±5 mm	600 200 225	IS 2185 (Pt-3):1984
Oven Dry Density (Kg/m3)	551−650	577	IS 6441 (Pt-1):1972
Compressive Strength (Avg. of 12 units, N/mm2)	(Grade 1) 4.0 Min. (Grade 2) 3.0 Min.	4.4	IS 6441 (Pt-5):1972

 

The firing of ammunition was executed using a 9 × 19 mm test barrel. A laser was attached to the top of the universal barrel to aid in pinpointing the impact location on the target. Standard 9 × 19 mm rimless, center fire type, ball cartridges (manufactured in 2015/ India) were used. The bullets were full metal jacketed round-nosed, featuring a lead-antimony core encased in gilding-metal (Table 2). The cartridges were tightly coned onto the bullets [8]. The muzzle velocity of the ammunition was measured using Intelligent IR Gate, with BMS Test Velocity LG software calculating velocities on a connected computer via Ethernet. Velocity was maintained at 430±15 m/s. Preliminary blank aiming was conducted, with reloading performed as needed to attain the desired velocity using a reloading unit. The characteristics of the 9 mm ball, both physical and performance-related, are outlined in Table 2.

 

Table 2. Observed Standard Parameters of 9 mm Round Nose, FMJ Projectile

FMJ Round Nose Projectile (9 mm)	Head Stamp
Length	14.89 mm
Weight	7.507 g
Base Diameter (Caliber)	8.96 mm
Tip Diameter	7.37 mm
Neck Diameter	8.52 mm
Propellant Filling in Case	0.450 g (approx.)
Muzzle Velocity	430±15 m/s

 

Analysis of Projectile Impact Data

A sample of 12 specimens underwent testing within the indoor environment of the Ballistics Range. The firing was done at a fixed 5 m distance from the target, with an IR light gate positioned 2.5 m from the barrel’s muzzle end to measure initial projectile velocity (gate data transmitted to a computer via Ethernet, with velocity calculations conducted through BMS Test Velocity LG software). A laser light integrated into the firing setup facilitated precise targeting of the sample’s center. The firing setup is visually represented in Figure 2 for reference.

 

Fig. 2. Graphical Representation of Firing Test Setup.

 

RESULTS

Firing and Impact Analysis

The firing process encompassed four angles: 0°, 15°, 30°, and 45°, achieved by adjusting the motorized moving table to the right. Each shot was directed at the center of the AAC block samples. In Study 03, identical AAC block samples were used for each angle of impact. Upon completion of each test, samples were removed from the motorized table, and comprehensive photographs of entry and exit holes were captured. The ballistic impact of the fired 9 mm projectiles on AAC blocks was subsequently examined.

Visual examination was followed by precise measurements of different parameters, including entry and exit hole diameters, depth of penetration (in instances without exit holes), and depth of crater formation due to scabbing. Vernier calipers and scales facilitated measurements. Bullet weight and dimensions were also recorded. During analysis, damaged areas of the samples were photographed, and fracture patterns were observed. These patterns were affected by sample thickness, surface area, and bullet energy loss upon impact [2, 9].

Energy Analysis and Calculations

For energy analysis, the bullet mass was considered constant. The study employed the kinetic energy formula:

$К.Е.=\frac{1}{2}m{v}^2$;   (1)

To determine impact energy, we used the following equation:

 $Е=m\frac{(v_x^2+v_y^2)}{2}$,   (2)

$v_x^2=v \cos \theta ,$

  $v_y^2=v \sin \theta -gt ,$

where

• m is the initial bullet mass in kg before surface impact
• v is the mean muzzle velocity in m/s determined using an IR gate 2.5 m from the barrel’s muzzle end
• and  represent the horizontal and vertical velocity components, respectively, of the moving projectile (m/s)
• θ is the angle of impact in degrees
• g denotes the acceleration due to gravity (assumed as 9.8 m/s)
• t is the time of impact (considered as 0.01 s)

For impact energy calculations, mean muzzle velocity was considered. Entry and exit hole diameters were recorded using vernier calipers, and GraphPad Prism 9.3.1 was used for graph plotting. As entry holes were not perfect circles due to friction and point of impact variance on fly-ash blocks, the best-fit ellipse method [5, 10, 11, 12] was used to accurately determine impact angles. Recovered bullets underwent analysis for deformations by measuring length, weight, base, neck, and tip diameters and comparing them against standard 9 mm bullet parameters.

Impact Energy Analysis

Using equations 1 and 2, we can establish a relationship between the angle of impact and energy loss. The mass of the bullet was considered constant at the point of impact, despite potential deformations occurring during the impact but not before that. The results are summarized in Table 3.

 

Table 3. Energy Impact and Loss at the Site of Impact w.r.t. Angle of Impact

Angle of Impact	Initial Energyi (J) (A)	Impact Energyii (J) (B)	Energy Lossiii (J) (C=A-B)	% Loss of Energyiii,
0°	694.02	694.02	0.00	0%
15°		694.04	-0.02	-0.003%
30°		693.86	0.16	0.023%
45°		693.82	0.20	0.029%

Note: (i) — distance from the muzzle end (2.5m); (ii) — at point of impact; (iii) — loss in K.E of bullet at time t = 0.01s.

 

From Table 3 and Figures 3 and 4 (created using GraphPad Prism 9.3.1), it is evident that increasing the angle of firing impact initially leads to heightened impact energy. Beyond a certain angle, however, there’s a noticeable decline in impact energy. The observed fluctuation in the curve might be attributed to reaching a critical angle where the curve transitions from a gradual increase to a steep decrease. This critical angle signifies the angle of elevation at which the projectile starts to ricochet. The absence of penetration at a 45° angle could be attributed to the maximum energy loss at this angle. Conversely, the most substantial damage occurred at a 0° impact angle due to no energy loss as there is no vertical velocity component, resulting in a non-parabolic trajectory. In contrast, any angle other than this exhibits a parabolic trajectory with a vertical velocity component influenced by gravity’s acceleration (–g). The projectile’s horizontal velocity component remains constant across all firing angles.

 

Fig. 3. Impact Energy.

 

Fig. 4. Energy Loss.

 

Determination of Angle of Impact from Entry Hole and Fracture Patterns

Bullets penetrated all samples at all study angles, creating entry holes with no damage visible at 0°and 15°. At 30° and 45°, some spalling was evident near entry holes, as illustrated in Figure 5. On the reverse surface, the bullet perforated with fragmentation/flying debris, except at a 45° impact angle, which showed no back damage/perforation (Figure 6). Table 4 depicts fracture patterns at entry and exit holes for different angles (0°, 15°, 30°, and 45°).

 

Fig. 5. Entry Holes at 5m Range at Different Angles — 0°, 15°, 30°, 45°, Respectively.

 

Fig. 6. Exit Holes at 5m Range at Different Angles: 0°, 15°, 30°, and 45°.

 

Table 4. Fracture Patterns Observed on AAC Blocks

Angle of Impact	Entry Damage	Exit Damage
0°	• Penetration • No loss in impact energy • Maximum bullet tip interaction with the target surface results in the minimum mean diameter for the entry hole	• Perforation • Mean diameter of the exit hole is greater than the mean diameter of the entry hole and is maximum
15°	• Penetration • Negligible loss in impact energy • Decreased bullet tip interaction with the target surface compared to 0°, and the mean diameter for the entry hole lies close to that of 0°	• Perforation • Mean diameter of the exit hole is lesser than the mean diameter of the entry hole and is lesser than that of 0°
30°	• Penetration and spalling • Visible loss in impact energy • Decreased bullet tip interaction with the target surface compared to 15°, and the mean diameter for the entry hole lies close to that of 15°	• Perforation • Mean diameter of the exit hole is lesser than the mean diameter of the entry hole and lies close to that of 15°
45°	• Penetration and spalling • Maximum loss in impact energy • Minimum bullet tip interaction with the target surface results in the maximum mean diameter for the entry hole	• No back damage/perforation • No exit hole was present

 

The mean diametric distances of entry and exit holes were measured and graphed (Table 5, Figure 7). Graphical analysis showed that the entry hole diameter increases with higher impact angles, attributed to a greater surface area of the bullet that comes in contact with the sample at the point of impact. Conversely, the exit hole diameter decreases with increasing angles. At 0°, the exit hole diameter is greater than that of the entry hole due to no energy loss at the point of impact, resulting in substantial damage. For other cases, the exit hole diameter progressively shrinks, with no perforation observed at a 45° impact angle due to increased energy loss at the point of impact.

 

Table 5. Diameter of Entry and Exit Holes w.r.t. Angle of Impact

Angle of Impact	No. of Shots Fired	Mean Diameter of Entry Hole (mm)	Mean Diameter of Exit Hole (mm)
0°	3	13.67	24.93
15°	3	14.42	12.36
30°	3	14.26	12.09
45°	3	22.93	No Perforation

 

 

Fig. 7. Diameter of Entry Hole vs. Exit Hole.

 

Crater depth measurements from the exit hole were 32.23 mm, 29 mm, and 19.90 mm for 0°, 15°, and 30° angles of impact, respectively. This shows that at 30°, the bullet traveled the farthest, causing the least damage due to the maximal energy loss upon impact, compared to 0° and 15°. Impact damage parameters of the major and minor axis (length, l, and width, w, of the elliptical shape of the entry hole) were measured and averaged (Table 6). The observed α and calculated θ values for the angle of impact indicated that the calculated angle provided an approximation for 90 – α rather than α, which is the actual angle of impact. Therefore, to understand the deviation and error in the known and calculated angles, a correction was introduced in the formula:

$\theta ={\sin }^{-1}\frac{W}{l}\times \frac{180}{\pi }$,   (3)

where, $\theta =90-\left({\sin }^{-1}\left(\frac{W}{l}\right)\times \frac{180}{\pi }\right)$ .

 

Table 6. Calculated and Known Angle of Impact

Tested parameters
Angle of Impact (α)	0°	15°	30°	45°
No. of Shots Fired	3	3	3	3
Mean Major Axis (mm)	15.83	15.64	16.85	29.37
Mean Minor Axis (mm)	15.75	14.99	14.48	19.99
Calculated Angle of Impact	84.24°	73.42°	59.24°	43.01°
Calculated Angle of Impact with Corrected Formula (θ' = 90–θ)	5.76°	16.58°	30.76°	46.99°
Error (θ' – α)	5.76º	1.58º	0.76º	1.99°
Standard Deviation	0.04	0.07	0.17	0.10

 

Angle of Impact Error and Regression Analysis

On calculating the error between known and calculated values for the angle of impact, a non-uniform variation becomes evident, with the maximum variation observed at 30° and the minimum at 0°. However, it is interesting to note that the least error occurs at 30°, whereas the highest error occurs at 0°, indicating a deviation from expected behavior. Conversely, angles 15° and 45° show consistent patterns without such anomalies. Employing both linear and non-linear regression on the data and plotting the non-linear regression (Figure 8), it was determined that the absolute error pattern aligned best with a quadratic equation. Utilizing GraphPad Prism 9.3.1, a statistically significant result was achieved through the least-squares fit (R² value 0.9922).

 

Fig. 8. Least Squares Fit.

 

Figure 9 shows the trajectory of the bullet upon impact, entering the fly-ash block and creating a small hole. This happened when fired from a 5m range at a 0° impact angle.

 

Fig. 9. Trajectory of the Impacted Bullet Fired at 0° from 5m Range.

 

Physical Examination of the Recovered Bullets

Bullet recovery was achieved at a 30° impact angle, while at 45°, recovery required cutting the AAC block with a disc grinder. The recovered bullets showed the right-hand rifling with a clockwise twist, characterized by one turn and six lands and grooves—indicative of a 9 × 19 mm test barrel. Deformation was observed at the tip without fragmentation. Table 7

 

Table 7. Analysis of the Recovered Bullets w.r.t. Angle of Impact

Angle of Impact	No. of Shots Fired	No. of Bullets Recovered	Dimensions of Bullet		Difference in Dimensions Before and After Firing (C = |A–B|)	% Loss in Dimensions
			Standard (A)	After Firing (B)
Length
30°	3	1	14.89 mm	14.02 mm	0.87 mm	5.84%
45°	3	1		14.51 mm	0.38 mm	2.55%
Weight
30°	3	1	7.507 g	7.430 g	0.077 g	1.03%
45°	3	1		7.445 g	0.062 g	0.83%
Base Diameter (Caliber)
30°	3	1	8.96 mm	8.86 mm	0.10 mm	1.12%
45°	3	1		8.95 mm	0.01 mm	0.11%
Ogive Diameter
30°	3	1	8.52 mm	8.87 mm	0.35 mm	4.11%
45°	3	1		8.66 mm	0.14 mm	1.64%
Tip Diameter
30°	3	1	7.37 mm	6.83 mm	0.54 mm	7.33%
45°	3	1		7.20 mm	0.17 mm	2.31%

suggests that at a 30° impact angle, more pronounced variations in various recovered bullet parameters were noted compared to the 45° angle.

DISCUSSION

The study’s results indicated penetration and perforation fracture modes at impact angles of 0°, 15°, and 30° while spalling occurred at 30° and 45° angles. No perforations were found at 45°, causing no damage to the AAC blocks’ backside. Notably, a similar study on steel fiber-reinforced concrete panels with varying thicknesses and fiber content reported consistent fracture patterns upon impact with a 9mm FMJ bullet at the center, indicating that damage was not observed beyond 40 mm [1]. These panels displayed excellent impact resistance due to their thickness and steel fiber content. However, in our study, the surface area of the bullet and the contact time between the bullet tip and the target surface emerged as influential factors in fracture patterns. Another study suggested that variations in fracture modes across different angles can be attributed to concrete sample composition, with energy absorption partially converting to fracture energy, thereby generating fracture surfaces [4]. This energy absorption trend was consistent with a study involving steel fiber-reinforced concrete panels, where impact energy absorption increased with greater thickness and fiber content [1].

Our analysis focused on bullet impact energy to understand the diverse fracture modes at different impact angles. Maximum damage and fragmentation were observed at 0°, associated with minimal energy loss. Conversely, at 45°, the projectile experienced significant energy loss, causing it to stop within the target without perforation. The impact energy analysis of the 9mm FMJ projectile on the target surface, fired from a 5m range, hinted at a possible critical angle attainment between 15° and 30°. A review study explored parameters for reconstructing crime scenes involving inanimate objects, highlighting distinct impact marks on different target surfaces. Concrete and wooden surfaces with round or elliptical entry holes and the ejection of surface materials from exit holes indicated the non-yielding behavior of these materials [12]. In our study, the measured entry hole diameters exceeded the caliber of the 9mm projectile, suggesting the non-yielding behavior of AAC blocks. It is interesting to know that a deviation was observed between the angle of impact calculated using the best-fit ellipse method and that observed on AAC blocks. Different studies utilizing best-fit ellipse and 2D ellipse methods were conducted to accurately determine the angle of impact and the repeatability of single bullet impacts. While the ellipse method effectively calculated impact angles up to 30°, an increase in impact angle tended to make impact sizes more circular than elliptical. This observation aligned with a previous study where circular impacts at 0° angle of impact exhibited reduced accuracy [10, 11]. Physical examination of the recovered bullets revealed characteristics of deformation without fragmentation. Additionally, AAC block components and rifling details were on the bullets, providing potential evidence for linking the bullets to gunshot holes and firearms at crime scenes.

CONCLUSION

Bullet recovery from crime scenes holds substantial evidential value aiding in determining firearm characteristics, caliber, and projectile features such as the number of lands and grooves. Furthermore, microscopic examination and characterization help in identifying the weapon type, facilitating bullet-weapon linkage. Our study focused on understanding the angular firing impact of a 9 mm round nose full metal jacketed projectile on AAC blocks. A noticeable trend was observed between the angle of impact and the entry and exit hole diameters. Greater impact angles led to larger entry hole diameters and smaller exit hole diameters, attributable to impact energy and bullet-surface contact area at the point of impact. At 0°, the bullet tip interaction with the target surface was maximized, causing significant impact at the exit point. The deviation in calculating the angle of impact using the best-fit ellipse method was addressed by modification, yielding a non-homogenous variation best explained by a quadratic equation plot. A similar study focused on the trajectory simulation of 7.62 mm/.308'' rifle bullets was previously conducted by one of the authors using numerical solutions of point-mass equations of motion [13].

Research Limitations

This pilot study’s limitations include its single impact on a single AAC block, with bullets not being recovered for 0° and 15° impact angles, preventing inferential analysis. For a more comprehensive analysis and crime scene reconstruction, studying a wall with multiple bullet impacts could be beneficial. Future research could delve into penetration, ricochet, and trajectory simulation, exploring combinations of ammunition and AAC blocks commonly used in criminal activities.

ADDITIONAL INFORMATION

Funding source. This study was not supported by any external sources of funding.

Competing interests. The authors declare that they have no competing interests.

Authors’ contribution. All authors made a substantial contribution to the conception of the work, acquisition, analysis, interpretation of data for the work, drafting and revising the work, final approval of the version to be published and agree to be accountable for all aspects of the work. Malika Singh conceptualized the idea and design of the work under the guidance of Dr. Richa Rohatgi and carried out the experiments under the supervision of Dr. Saurabh Kumar. Malika Singh and Dr. Saurabh Kumar formulated the results. Dr. Richa Rohatgi critically evaluated interpretation of results and contributed in submitting limitations and scope of study. Dr. Sanjay Gupta critically reviewed the article and suggested intellectual inputs to improve on content and writing. All the authors read and approved the final version of the manuscript before publication, agreed to be responsible for all aspects of the work, implying proper examination and resolution of issues relating to the accuracy or integrity of any part of the work.

Acknowledgements. The authors are thankful to the facilities and experimental support provided by NFSU Delhi Campus and Ballistic Research Centre and Testing Range (BRCTR), National Forensic Sciences University (NFSU) Gandhinagar. The guidance and support offered by Shri S.G. Khandelwal, Head, BRCTR, Mr. Abhijitsinh Parmar, Scientific Assistant, BRCTR, and Ms. Kruti Panara, BRCTR, during the ballistic trial. We are thankful to everyone who rendered their help throughout the research process till the manuscript preparation. Token of thanks to Mr. Ravi, Civil Engineer, who helped procure the samples.

About the authors

Malika Singh

LNJN NICFS National Forensic Sciences University

Email: singhmalika98@gmail.com
ORCID iD: 0009-0005-4749-1622
India, New Delhi

Dr. Richa Rohatgi

LNJN NICFS National Forensic Sciences University

Email: rrohatgi2020@gmail.com
ORCID iD: 0000-0001-5514-953X

MD, Dr. Sci. (Med.), Assistant Professor

India, New Delhi

Saurabh Kumar

National Forensic Sciences University – Ballistics Research Centre & Testing Range

Author for correspondence.
Email: saurabh.kumar@nfsu.ac.in
ORCID iD: 0000-0001-8442-1096

MD, Dr. Sci. (Med.)

India, Gandhinagar

Dr Sanjay Gupta

Department of Forensic Medicine & Toxicology, All India Institute of Medical Sciences

Email: drsanjaymdfm@gmail.com
ORCID iD: 0000-0003-3829-3155

MD, Dr. Sci. (Med.), Professor

India, Rajkot (Gujarat)

References

Supplementary files