These tables provide theoretical data on the results of naval shell impacts against armor in the World War II context. The belt penetration figures are derived from Nathan Okun's FACEHARD program with certain conditions in mind - a 90° target angle; the shell's cap and windscreen are intact; use of new-gun muzzle velocities; and specifics of plate placement (surfaces being vertical and flat and exterior rather than interior or curved or inclined, factors which would necessitate different calculations).
Real-world gunnery involved innumerable additional factors such as barrel erosion, premature shell detonation, quality control in manufacturing, roll motion of the target - and on and on. Consequently, these tables suffice only as a measure of probability with no pretense to absolutes. Two shells may strike under seemingly identical conditions yet produce widely disparate results.
The top row of each table indicates the range in thousands of yards. The first column indicates the armor type and the impact results. Note: A ballistic limit can be expressed as the velocity needed to defeat a given thickness of plate or, as in these tables, as the maximum thickness of armor a shell can defeat at a given velocity. The two go hand in hand.
Partial Penetration - The forward half of the shell penetrates the armor while the rear half is rejected; or, for hits over 45° obliquity, the nose and upper body are rejected while the broken lower body penetrates - only if the shell breaks, of course, but most will at such a high obliquity below the Naval Limit.
Holing Limit - The shell makes a caliber-width hole entirely through the plate without enough of its body weight completely penetrating to meet the Naval Limit definition. Hits that fail to meet this criterion can still cause limited damage, usually more localized to the area behind the struck plate, perhaps causing shock effects only.
Naval Limit - If undamaged, 100% of the shell's body weight achieves complete penetration (ignore the AP cap and windscreen); or in the case of a broken shell, 80% of the body weight penetrates. Beyond this, the shell's post-impact condition remains undefined; it may be capable of exploding, or it may not.
Effective Limit - The projectile will usually retain intact its explosive filler cavity, a seated base plug, and a working fuze. Other damage to the shell is not addressed in this definition. The Effective Limit may be at a velocity equal to or above Holing Limit, or it may be impossible at any striking velocity under the given impact conditions.
Hits at velocities below the Holing Limit are largely undefined as to their effects on the projectile, with few being able to remain intact and even these usually only at a rather low obliquity.
The tables also provide information on the penetration of horizontal homogenous armor, which is subject to a higher degree of uncertainty. No system currently exists for quantifying the effects of differing shell forms for specific predictions. Nathan Okun's M79APCLC program presents the best model available. It is most accurate in predictions for British and German shells, while Japanese and American shells suffer slightly. The figures in these tables involve the following inputs: British armor, quality 1.0 with 25% elongation; US armor, quality 1.0 with 25% elongation; Italian armor, quality 1.0 with 17.1% elongation; German armor, quality 1.0 with 18% elongation; Japanese armor, quality 0.97 with 23% elongation.
The following guns used nose-fuzed projectiles, a type not included in FACEHARD: The US 5"/25; and the Japanese 5"/40, 5"/50, and 3.9"/50. According to Nathan Okun, a nose-fuzed projectile would penetrate 0.2 calibers of homogeneous ductile plate (obliquity not important when filler explodes in most cases) or 0.3 calibers of a face-hardened KC-type plate. Thus a typical 5" gun firing a nose-fuzed projectile would penetrate 1.0" of homogeneous armor or 1.5" of face-hardened armor throughout its range, with most of the penetration from the explosive force of the burster rather than kinetic energy.
We have two men to thank for these tables. Rob Lundgren performed each penetration calculation posted in these tables, a task demanding many, many tedious hours. The calculations were possible only through Nathan Okun's monumental achievement, the FACEHARD formula, the result of a decades' long investigation into armor and shell performance.
- 27 November 2009