There is some evidence that a modified 36 cm and 46 cm Type 91 AP projectile, called the Type 1, with a slightly longer and more pointed windscreen and a single wide British-style driving band – similar to pictures of known experimental 48 cm AP projectiles that were tested during World War II – was introduced at some time during World War II, but there are no photographs of these projectiles, to my knowledge. The 46 cm size, if actually produced, may have never actually been issued to YAMATO or MUSASHI, though some 36 cm Type 1 AP shells seem to have been used during World War II. These changes did not affect any shell performance criteria.
The Cap Head has no function whatsoever at over 45̊ obliquity against homogeneous armor, being knocked aside immediately on impact. Against face-hardened armor, the Cap Head shatters, but it seems to provide some protection to the 15.5 cm and 20.3 cm uncapped projectiles, allowing holing of these plates more easily, but not preventing these projectiles themselves from shattering when they try to penetrate through the holes just made. The Cap Head and windscreen are knocked off by impact with practically anything, including the surface of the ocean, which is what it was designed to do, so as to allow the tapered flat nose to cut through the water with minimum deflection or speed loss and enable the projectile to hit the target deep underwater at a longer miss distance, which it actually only did once, to my knowledge, against the US Navy light cruiser USS BOISE.
The 15.5 cm Type 91 Cap Head design was a virtual copy of the 36 cm Cap Head in outer contour, but this shell had a much larger explosive cavity and the AP cap and nose were fuzed together (made in one piece from the start). It used the same rather blunt button-shaped Cap Head. When thicker armor was expected in foreign ships – though obviously not the use of face-hardened armor in cruiser-size ships – a thicker, more-pointed Cap Head was added to the 20.3 cm design to improve penetration into homogeneous ductile armor at low obliquity. Otherwise, weight considerations imply that the 15.5 cm design was more-or-less identical to the 20.3 cm design. The 20.3 cm size was found to have excellent homogeneous armor penetration ability in post-World War II US Navy tests of this projectile and the last German "L/4.4" APC projectiles in comparison to US Navy AP projectiles.
The thicker, more-rounded Cap Head used with the capped 46 cm Type 91 projectile (similar to the 20.3 cm Cap Head, but blunter) was part of an effort to improve the projectile’s oblique impact (30-35 degrees) penetration ability against face-hardened armor, but the improvements were rather small, if any, so I do not separate the 46 cm design from the earlier capped 36 cm and 41 cm designs in this regard. None were up to most foreign designs of the same period in AP capability, since the underwater (diving) hit concept and minimum drag for maximum possible range body shape seems to have absorbed all innovation, including retaining the circa-1921 British 15" Mark V AP projectile’s 20-degree-obliquity-versus-0.67-caliber-VC-plate test specs adopted along with the last British APC shells of that type bought at that time. Most foreign World War II-era AP projectile tests required - or, at least, could pass - a 30-degree or, in the US Navy case, up to 35/40-degree obliquity test against up to caliber-thickness face-hardened plate with an intact explosive cavity and fully-functional base fuze after penetrating the plate.
"S.O." means "Secant Ogive" nose shape. This is a single circular arc forming the outer contour, but here the arc center is lower and closer to the centerline than if the radius line to the joint of the arc and the cylindrical body or other adjacent projectile part’s edge were at right angles to the centerline (= a "Tangent Ogive" shape), so that the nose arc meets the upper body cylinder or that other part’s edge with a distinct crease or shoulder at the joint (Tangent Ogive shapes "feather" at the joint perfectly, with no shoulders or creases there). Distances given here are from projectile bottom up to the given center level along the projectile centerline.
These measurements are from actual blueprints made after World War II by cutting Japanese projectiles apart at either the U.S. Naval Proving Ground or a U.S. projectile manufacturer’s steel plant and making measurements.
