I believe a 6.5 Swedish Mauser, a 7mm Spanish Mauser, a 7.65 Belgian Mauser, an 8x57 German Mauser, a 303 Enfield or a 30/06 Springfield would all go right through that tiny piece of armor. It probably would stop pistol bullets or shrapnel.
They could just aim below the “armor” and hit him in the nads.