## 2 Experimental Study
### Experimental Setup  
Considering the test cost, the experiment was conducted two times. The projectile with the lower speed was fired with a 30 mm smoothbore gun, and the field layout is shown in Figure 1a. The projectile with the higher speed was fired with a 60 mm smoothbore gun, and the field layout is shown in Figure 1b. The projectile body was launched by a 30 mm gun with a nylon airlock ring, as shown in Figure 2a. The projectile body was fired with a 60 mm gun with a nylon sabot and the projectile surface was painted to facilitate better capture of the projectile with the high-speed camera, as shown in Figure 2b. The rock target was a 600 mm \(\times\) 600 mm square area with a length of 1 m. The rock target was placed in a steel ring with a diameter of 1 m and C40 concrete was filled between the target and the steel ring, as shown in Figure 3. The projectile material was 30 CrMnSiNi2A. After heat treatment, the dynamic yield strength exceeded 1 GPa, and its composition distribution is shown in Table 3.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimen Number** & **Strain Rate/(s\({}^{-1}\))** & **Specimen Size/mm** &
\begin{tabular}{c} **Compressive** \\ **Strength/MPa** \\ \end{tabular} \\ \hline Red Sandstone-1 & \(2\times 10^{-4}\) & \(\varnothing 41.3\times 79.6\) & 59.12 \\ Red Sandstone-2 & \(2\times 10^{-4}\) & \(\varnothing 40.5\times 79.5\) & 58.28 \\ Red Sandstone-3 & \(2\times 10^{-3}\) & \(\varnothing 41.9\times 79.4\) & 50.89 \\ \hline Limestone-1 & \(2\times 10^{-4}\) & \(\varnothing 42.5\times 80.0\) & 110.79 \\ Limestone-2 & \(2\times 10^{-4}\) & \(\varnothing 41.9\times 80.0\) & 114.48 \\ Limestone-3 & \(2\times 10^{-3}\) & \(\varnothing 42.3\times 79.9\) & 126.80 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Groups of quasistatic compression tests.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Specimen** & \
\begin{tabular}{c} **Compression** \\ **Speed/(m\(\cdot\)s\({}^{-1}\))** \\ \end{tabular} & **Diameter/mm** & **Thickness/mm** &
\begin{tabular}{c} **Tensile** \\ **Strength/MPa** \\ \end{tabular} \\ \hline Red Sandstone-1 & \(2.0\times 10^{-7}\) & 49.08 & 25.90 & 1.51 \\ Red Sandstone-2 & \(2.0\times 10^{-7}\) & 49.36 & 25.80 & 1.11 \\ Red Sandstone-3 & \(2.0\times 10^{-7}\) & \(50.11\) & 24.89 & 1.83 \\ \hline Limestone-1 & \(2.0\times 10^{-7}\) & 48.24 & 24.71 & 3.77 \\ Limestone-2 & \(2.0\times 10^{-7}\) & 48.58 & 24.74 & 4.05 \\ Limestone-3 & \(2.0\times 10^{-7}\) & 49.44 & 24.67 & 5.49 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Splitting test grouping.  
Figure 1: Test site layout: (**a**) 30 mm-diameter smoothbore gun; (**b**) 60 mm-diameter smoothbore gun.  
Figure 3: Test target: (**a**) red sandstone target; (**b**) limestone target.  
Figure 2: Test projectile: (**a**) 30 mm smoothbore gun test projectile; (**b**) 60 mm smoothbore gun test projectile.  
To study the differences in penetration resistance of different rocks, three test schemes for the penetration of red sandstone and limestone at different speeds were designed, as shown in Table 4. In the experiment, a high-speed photography system was placed in front of the target to measure the actual impact velocity and target attitude of the projectile.