Alberto Virtuani wrote:
Could anyone please elaborate on the APCBC shells ?
I'm not an expert at all, but I understand APCBC shells are simply shells fitted with a ballistic cap AND a windscreen (aerodynamic cap),
Thorsten and Tommy can probably provide more detailed information.
Caps were developed to help AP shells survive the impact against armour. Indeed, Krupp Cemented armour was developed specifically to shatter AP shell on impact. Harder and larger caps were developed in turn. Better armour formulations were developed in turn. Many WW1 and earlier shells had comparatively soft caps. Soft capped shells still had problems surviving impact with face hardened armour.
During the inter-war years harder, but also larger and heavier caps were the trend. For example, the difference between the 2110 lb and 2240 lb USN 16" shells was the size and the weight of the cap. The Germans found that a larger and heavier cap does help aid penetration. I have a late war US document that describes the purpose of the cap was both to protect the shell and to aid penetration. It states that upon impact, the cap helps to stress the armour and helps the shell dig in before it is destroyed. Then once penetration is started, and the cap is destroyed, the head of the shell can better push through the remaining armour like a "flat nosed punch." Removing the cap reduces the penetration capabilities of the shells and causes the shell to shatter against heavy face hardened armour.
The Germans found that removing the cap is also good in case of deck hits. Removing the cap reduces the weight and therefore the kinetic energy of the shell. Not only does the weight of the cap go away but also the momentum/velocity of that portion of the weight too. With larger and heavier caps this reduction is more significant. The weight of the cap can be more than 13% of the overall shell weight. The Germans found that there was a further significant reduction of penetration of de-capped shells if the deck armour had a tensile strength at least or exceeding 80kg/mm2 (130ksi)
Penetration at acute obliqueness is mostly the function of the shape of the head of the main body of the shell that the cap is fitted over. A sharply pointed head shape is more likely to deflect or "scoop" at more acute striking angles, but striking closely to the normal the penetration is greater. British and Italian shells had relatively sharp pointed shells of 1.4 calibers radius. American shells were much rounder shaped, being almost hemispherical in nose shape. late in the war the USN changed to an even more blunt nose shape to improve oblique penetration further. The Germans found that a nose shape of 0.93 caliber radius provides almost as good oblique performance as a hemispherical head shape but without the large trade off in normal striking performance. The rounder the nose shape, the easier it is to knock the cap off, is another trade off.
The British tested captured Italian 15" shells along side their own. They found that Italian shells were highly likely to scoop in the case of oblique hits , but that the fuse was more likely to survive the "base slap" intact.
Base slap was major problem. When a shell strikes at an oblique striking angle it will momentarily change trajectory more away from the normal which causes the base of the shell to slap against the plate during penetration or when scooping, usually distorting or breaking the fuse cavity, destroying the functionality of the fuse. The longer the shell, the more likely this was the case. The Italian fuse cavity was better protected against base slap because it was located deeper up into the shell.