Penetration results are reported for four low strength materials attacked by a non‐precision, copper‐shaped charge jet. The materials are methanol, Plexiglas, glycerol, and mild steel. The results are not entirely consistent with the recent compressibility‐based predictions by B. S. Haugstad and O. S. Dullum [J. Appl. Phys. 52, 5066 (1981)] and by N. A. Zlatin and A. A. Kozhushko [Combust. Explos. Shock Waves USSR 16, 559 (1980)]. Methanol and Plexiglas have extraordinary penetration resistance due to a combination of shock wave formation, plastic flow resistance, vaporization, and decomposition. The more chemically stable glycerol has simpler behavior that is similar to mild steel.