## 2 Experimental Details
### Penetration Test with 7.62 AP Bullet  
The bullet penetration test has been performed in the armament testing center of Defence Metallurgical Research Laboratory, Hyderabad (India). A schematic diagram of the bullet penetration test is shown in Figure 1.  
The schematic diagram of experimental setup use for bullet penetration is shown in Figure 1. A 7.62 mm armor-piercing hard steel core projectile (7.62 AP) covered with a copper case was used in the ballistic experiments. The projectile core without copper case has dimensions of \(\Phi\)7.62 \(\times\) 11.43 mm (Figure 2) and a mass of 5.34 g, while the core with the copper case has a mass of 10.4 g [12]. Bullet shots were fired on the target plates (base metal, weldment-1 and weldment-2) with the recorded impact velocity of 820 m/s.  
The target plate dimensions are 200 \(\times\) 150 \(\times\) 25 mm. The distance between the target plate and riffle muzzle is 10 m in the present experiment. At the time of the firing, the gun with accessories like receiver and firing mechanism is fastened on a mounting with holding device and adaptors for providing stability to the 7.62 AP bullets. The gun and target plate planes are leveled with 1.5 m distance above the ground level. The 7.62 AP bullet is attached with a cartridge which is put in the firing mech  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Material** & **C** & **Si** & **Mn** & **P** & **S** & **Cr** & **Ni** & **Mo** & **N** & **B** & **Fe** \\ \hline Base metal & 0.31 & 0.265 & 0.86 & 0.019 & 0.007 & 0.60 & 0.82 & 0.24 & - & 0.005 & Balance \\ Weldment-1 & 0.06 & 0.58 & 0.85 & 0.017 & 0.012 & 0.04 & 0.03 & 0.015 & 0.018 & - & Balance \\ Weldment-2 & 0.05 & 0.51 & 1.18 & 0.018 & 0.06 & 18.03 & 9.45 & & 0.04 & - & Balance \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical composition (wt. %) of test samples under investigation [3].  
Figure 2: Schematic diagram of 7.62 AP Bullet steel core [13].  
power for movement of the bullet after triggering the gun. The rifling in the bore section helps to spin the bullet inside the bore and reduce the wobbling.  
There are two velocity probes. The first placed before the target for measuring the impact velocity at the distance of 6 m from the gun barrel, and the second is situated behind the target at the distance of 0.2 m for measuring residual velocity. The velocity probes are connected to the multi-channel electronic timer for measuring the time in travel a fixed distance. Aluminum foils are used to fabricate the velocity probes and it is placed in a wooden frame with cardboard. In the present investigation, the target plates thickness is 25 mm, and it cannot be fully penetrated by 7.62 AP bullets. Further, the results come in the form of depth of penetration (D.O.P).  
[PERSON] et al. [13] observed that, when only the steel hard core was used as a projectile, the ballistic limit dropped by 3-5%, which indicates that modeling only the steel hard core in FE simulations of a bullet penetrating armor steel targets is conservative. Hence, in the present numerical simulation's investigations only steel core of 7.62 AP bullet has been used.