The Armor Thickness Necessary to Decap an APC Projectile

by Nathan Okun
Updated 27 October 1999

Armor penetration requires a rather lot of information, but decapping of the projectile by breaking the rather weak solder and/or mechanical bond between the nose and cap base is VERY, VERY SIMPLE:

(1) 0.08-0.08049-caliber thickness of ANY KIND OF HOMOGENEOUS IRON OR STEEL PLATE has a 50% chance of decapping ANY KIND of capped projectile over 40mm in diameter under ANY impact condition, penetrating or not.

(2) 0.0805-caliber and up thickness ALWAYS decaps the projectile, penetrating or not.

(3) Increase these two thicknesses by 50% for 40mm and smaller capped projectiles for some obscure reason.

(4) A base-fuzed Common projectile with a HOOD to attach a windscreen (a U.S. Navy WWII "Special" Common projectile, for example) acts like the HOOD is a soft AP cap in this regard, but the HOOD does not protect the nose from shatter against face-hardened armor, though.


I have lots of tests by the U.S. Navy, U.S. Army, British Navy, and British Army during WWII with normal and specially-made soft and hard AP caps to verify this.  It is one of the ONLY simple rules in this entire subject!!!!!

Editor's Note:

The USN Iowa and South Dakota class battleships have an internal inclined main armor belt.  What isn't well known is that they also have a shell plating outside of this belt that acts as a decapping plate.  On the South Dakota's, this shell plating is 1.25" thick (3.2 cm) and on the Iowa's it is 1.5" thick (3.81 cm).  Using Nathan's formula above, the South Dakota's plating would be sufficient to decap any projectile up to 15.5" (39.4 cm) and the Iowa's plating would be sufficient to decap any projectile up to 18.6" (47.3 cm).  This would imply that the Japanese Type 91 18.1" (46 cm) APC projectiles fired by the Yamato would be decapped by the Iowa's shell plating before they reached the main armor belt.  Decapping an AP projectile greatly decreases their armor-penetration ability against face-hardened naval armor (unprotected projectile nose now shatters into pieces) at under 45° impact obliquity angle. Changes against softer homogeneous naval armors (decks, for example) are small--primarily due to the loss of the cap's weight.

Tony DiGiulian


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