You can adjust effective penetration depth by the "density" (read, mass per unit cross-sectional area) of the projectile. You could go anywhere from an osmium

You can adjust effective penetration depth by the "density" (read, mass per unit cross-sectional area) of the projectile. You could go anywhere from an osmium needle through to a Mylar disk. Another relevant XKCD: https://what-if.xkcd.com/20/ https://what-if.xkcd.com/20/