When an AP projectile hits an armor plate so hard that the hard metal at its tip will shatter due to shock or, slightly later, compression against the plate surface as the inertia of the lower body builds up (the shock effect I call primary shatter and the compression effect on the surface I call secondary shatter), its penetration ability and, usually, its explosive effectiveness are altered markedly (below about 55° obliquity, the penetration is degraded - plate thickness gets a 30% bonus increase effect at normal (right angles) and this bonus slowly decreases to zero at about 55° obliquity from the normal - though above this obliquity, the suppression of ricochet actually makes penetration easier by the broken pieces).

To try to prevent the loss in penetration ability at low obliquity, various things have been tried. Against face-hardened armor, the solid iron or steel "Armor-Piercing Cap" (AP Cap) was developed, since projectile shatter occurred against this extremely hard-faced armor at virtually any striking velocity where penetration was even remotely possible (the AP Cap also works against softer homogeneous armor at high velocities, but usually naval projectiles are going too slowly to shatter on homogeneous naval armor, unlike close-range anti-tank projectile fire). If the shell body and AP cap were in one piece, there would be nothing to stop the breakage of the cap/nose from spreading downward until it reached the lower edge of the hard nose near the forward bourrelet, which was what the sacrificial cap was there to prevent - you wanted an intact hard nose after the initial impact shock was over with, as otherwise the projectile nose and, usually, middle body collapsed like a watermelon dropped from a second-story window onto a concrete sidewalk!!

There are two types of AP Caps: Soft and Hard. I divide them at about 300 Brinell Hardness, where the usual cap material begins to break up immediately rather than deform first on impact.

Soft AP caps work by flattening out on the plate like a doughnut surrounding the nose and when the nose itself hits the plate the shock wave created by the hard nose meeting the hard face surface now has some place to go in the sideways direction (it always could travel down the length of the projectile in that direction). Without the soft cap material to absorb the sideways shock, the shockwave there would have to reflect back into the nose and the nose would crack and blow outward like an over-inflated tire! With the soft cap ring there, the reflection occurs at the outer boundary of the cap doughnut instead and the cap, not the projectile nose, blows away sideways, spraying the surrounding region with a ring of fragments, but leaving the projectile nose intact and ready to push through the plate (note that other kinds of projectile damage can still occur, but none as dramatic in its effects as shatter). Soft caps unfortunately begin to be torn free prior to functioning at obliquities of only 15° and by just over 20° obliquity have virtually no use whatsoever, as any kind of air gap between the cap and the projectile nose nullifies its benefits. Also, a few WWI-era and almost all WWII-era face-hardened armors (except Japanese VH) improved their toughness so much due to improved metallurgy that even after the projectile nose has been saved from primary shatter, the plate refuses to cave in and the crushing effect of the middle and rear projectile weight squeezing the nose against these plates causes the nose to split open prior to penetrating, with virtually the same effect as the shock-induced shatter - this is my secondary shatter of soft-capped projectiles, which is rare against most WWI-era armors, as mentioned (U.S. Midvale Non-Cemented Class "A" armor and Austro-Hungarian Witkowitzer KC-type armor are the only ones that had this "SOFTSHAT" property in WWI, to my current knowledge), since these older plates were brittle and if primary shatter didn't work, the plate caved in first, relieving the pressure on the projectile nose and preventing secondary shatter from occurring.

Hard AP caps (German and Austro-Hungarian post-1911 caps and British post-Jutland "Greenboy" caps and virtually all post-WWI-developed AP Caps) have the same benefits as soft AP Caps, but add the following:

  1. Since they do not deform, obliquity does not mean much to hard AP Caps and as long as they are large enough to hit the plate prior to any part of the projectile nose, they always work.
  2. They are hard enough to destroy the plate face as the cap is crushed, so they dig a pit into the plate face prior to the projectile nose impact (soft AP caps did virtually nothing to the plate prior to the nose hitting it). This acts like a center-punch to hold the nose in the hole and it destroys the hardest part of the plate and it makes the plate thinner at the impact site when the nose hits the plate shortly afterwards. This makes a thicker hard cap have some advantage over a thin hard cap, so hard AP Caps tended to be much thicker and heavier than soft AP Caps did - there is of course a point of diminishing returns here, but U.S. WWII 6" Mark 35 AP projectile caps averaged about 20% of the total projectile weight in the latter half of WWII and U.S. 8" Mark 21 AP projectile caps averaged 17% (and this 8" projectile was the "super-heavy" type so its AP Cap was really thick!!). Most capped AP and Common projectiles in WWII had 10-16% AP Caps, averaging about 12%.
  3. The U.S. Navy also found that making the AP Cap as hard as possible entirely through - assuming that the projectile nose and body was strong enough, which U.S. AP projectiles were in spades - would cause even greater destruction to the plate face, so the last versions of the 6" Mark 35 (Mods 9 and 10) AP projectile and of the 8" Mark 21 (Mod 5) AP projectile had their super-thick AP Caps hardened to 650-680 Brinell - harder than the face surface of most KC-type armor plates!! - entirely through to the surface of the nose (!!!) with extremely good results.

If you read old books, they talk about things like the "Shoulder Support Theory" (where the inertia of the cap material to the sides of the nose held the nose together on initial impact) or the "Lubrication Theory" (where the soft cap acted like a kind of flux to make the forming of the hole easier). These totally wrong theories did allow soft AP Caps to still function, as the caps that resulted were still tightly clamped to the nose and flattened out properly - the concept of shock waves simply was not grasped by the projectile or armor manufacturers, even though there were other scientists and engineers who had explained it correctly for years in other fields (it makes you wonder what these so-called ballistic experts read about during and after their schooling; it sure wasn't about the reaction of steel to high-speed shock!!).

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12 January 2000