The armor protection of USS ARKANSAS was made, as were all ships of its era, of patches of armor from all of the three large U.S. naval armor steel manufacturers--Carnegie (later Carnegie-Illinois and, still later, U.S. Steel), Bethlehem, and Midvale. This means that different spots on the ship have different protection levels, since there was a significant difference between the face-hardened U.S. Navy Bureau of Ordnance Class "A" armors made by these manufacturers through 1923 in many cases (in the mid-1930's, when armor was again made, the differences became rather minor). The ARKANSAS was built during the worst part of this difference era:
1. The Carnegie-Krupp Cemented (CKC) Class "A" armor was very similar
to slightly improved (tempered) German Krupp KC armor.
2. Bethlehem at this time was experimenting with its Bethlehem Non-Cemented (BNC) high-Carbon, high-Chromium, low-Nickel Class "A" armor, which was very brittle and of minimum acceptable quality (Bethlehem quit making it shortly thereafter).
3. Midvale was also experimenting with its Midvale Non-Cemented (MNC) Class "A" armor, which also used a high Carbon content, but normal Krupp armor levels of Nickel and Chromium. By the way, "Non-Cemented" means no thin (circa 1" thick) "Harveyized" or "cemented" (carburized) surface layer so the maximum hardness was less for such armors. MNC also used an immensely thick face that only softened to the tough back level at about 80% (!!) of the way from the face surface to the back surface, which KC and CKC used only a 33-35% face/transition layer thickness (I do not know what BNC used in this regard). MNC was also somewhat brittle (though not nearly as bad as BNC) and was dropped by Midvale just after BNC was dropped by Bethlehem.
After WWI, a lot of testing was done by the U.S. Navy for the "Build the Limit" battleship/battle-cruiser program and many old plates were retested with the new, greatly-improved "Midvale Unbreakable" or "Midvale 1916" (specification date) soft-capped U.S. Navy AP projectiles, especially the new 870-pound 12" Mark 15 Mod 6 AP projectile. CKC was found to be slightly better than captured German Krupp KC (Krupp KC was too brittle since Krupp never changed from testing his armor using uncapped AP projectiles, which were the kind that existed in 1894 when he first made KC armor--talk about reactionary!) using these new projectiles, too. BNC was again found to be barely above the minimum resistance allowed (failed here too!). But MNC armor was found to completely shatter (!!!!!) the new projectiles as though they had no AP caps!!!! The super-thick face caused this and they had a lucky break in the tempering process they used (without knowing it, they had found the best tempering temperature possible to toughen their armor even with the excessively brittle, excessively deep face layer). This made the armor 30% better (equal to a plate 30% thicker!) than CKC against soft-capped 12" projectiles and they made some more armor of this type for one of the canceled battleships or battle-cruisers (do not know which). However, when tested with the newer, larger 14" and 16" Unbreakable projectiles, the projectiles still shattered, but the resistance of the plate dropped precipitously--a huge scaling effect caused by the too-thick brittle face which fails by shearing and breaking instead of the more gradual bending, stretching, and tearing, as the tough back layer does. Against the 16" Midvale Unbreakable projectile size, the plates were no better than old Harveyized Nickel-steel against unshattered projectiles (really b-a-d by 1921 standards!). Against 8" projectiles, the scaling would work in reverse to make the plates even more God-like against WWI-era soft-capped AP projectiles.
Thus, parts of the ARKANSAS (which parts?) are super-protected (MNC) against most WWI-era guns up to about 12" in size, but other parts are average (CKC) and still other parts are poor (BNC). "Spin the dial and takes your chances!"
Improved projectile and hard AP caps that became standard after WWI
eliminated most of the benefits of the thick face even against 12" projectiles,
but, unfortunately, U.S. manufacturers during WWII continued to try to
break the continually-improving U.S. AP projectiles (an ultimately futile
effort) and made their Class "A" armor face thicknesses too heavy (circa
55%), causing a mild form of the same kind of scaling problem that the
MNC caused in the 1921 tests--against guns up to 8" it was the best, but
as projectiles got larger than this, there was a gradual decrease in relative
resistance with WWII U.S. Class "A" armor compared to foreign types that
had thinner face layers (they also had differences between each other,
but to a much smaller degree). Complicates things!
The problem with German WWII naval ammunition can be stated in one word: K R U P P
Krupp manufactured all German naval projectiles and most naval armor since prior to the year 1900.
In 1911, Krupp did a very progressive thing: They eliminated Lyddite ("Gr.f.88" to them) from their anti-ship projectiles as substituted block TNT (layered and covered in felt and paper and with a thick wooden plug at the upper end of the explosive cavity). By doing this they eliminated the premature detonation on impact with face-hardened armor that the too-sensitive Lyddite (trinitrophenol or picric acid are alternative chemical names) caused and this allowed delay-action fuzes to be made (the first impact-initiated delay-action fuzes for high-explosive-filled projectiles ever used, to my knowledge). During WWI, this turned out to be a good idea, WHEN IT WORKED!
The major problem was that TNT was rather insensitive (which is why it could be so easily kept from exploding on impact), which meant that it was hard to set off "high order" ("full detonation" in British terminology)) by the very tiny booster/gaine/magazine explosive at the tip of the fuze which directly blasted against the filler and set it off (also Lyddite at the time, as it was the most powerful "safe" explosive known). Nose-fuzed HE projectiles merely used a much larger booster to make sure of enough "kick" to set off the filler, but the base fuze in AP/base-fuzed Common projectiles only could have a small booster if the fuze was not to break due to the high impact shock against armor. This caused a large number of duds, partial detonations, and even low-order "deflagrations" or "bursts" rather than detonations on impact.
Krupp and the U.S. Navy, which simultaneously introduced its even more insensitive filler Explosive "D" ("Dunnite" or chemically ammonium picrate) in 1911 into almost all projectiles, replacing the previous black powder fillers, simply lived with the problem as the price of getting the projectile to detonate after it had pierced an armor plate, rather than while still penetrating it in most cases, as Lyddite caused to happen (U.S. projectiles did not use delay-action fuzes until after WWI, but still got a slight improvement over Lyddite when penetrating heavy armor)--Lyddite was a very reliable explosive, very powerful, it merely liked to detonate high order too soon! TNT and Explosive "D" were not as powerful as Lyddite, either, though "good enough for government work" in this case!
After Jutland, for base-fuzed Common and AP-type projectiles the British also adopted TNT (cast rather than in pre-shaped blocks and with a small pad in the cavity tip rather than a wood block) and the even more insensitive explosive "Shellite" (a mixture of Lyddite with 30% of a much weaker, insensitive explosive dinitrophenol, which was similar to Explosive "D" in power and almost as insensitive--no cushions in the cavity needed with them!). They also adopted an improved delay-action fuzes based originally on the rather poor German (Krupp) design and later on their own designs, but they also had the problem with the Lyddite boosters that everyone else had who adopted insensitive explosive fillers in AP-type projectiles.
After WWI, things were rather different for Krupp military products. In 1928, the British and U.S. Navies both hit on totally different safe methods of using the very powerful, but very dangerous, explosive "tetryl" in their boosters in tiny amounts that could reliably set off TNT, Shellite, and Explosive "D". Japan went another route and in 1931 adopted the slightly less sensitive, slightly less powerful filler "Type 91 Explosive" (trinitroanisol or TNA) in place of Lyddite (after years of a failed attempt to cushion Lyddite to work), using cushioning in the cavity that took up to 40% of the volume of the cavity--but it was reliably detonated by a Lyddite booster (a REALLY stubborn "so there" mental fixation if there ever was one)!!
Krupp improved the mechanical portions of its base fuzes, but, to my knowledge, never changed its booster from the pre-WWI Lyddite type, with identical results, from WWII battle reports. Krupp AP and base-fuzed Common projectiles were in some ways significantly improved over their WWI ancestors, but not very much in other important ways (same half-caliber KC armor at 30° obliquity acceptance spec, same "tuna can" base plug design, same nose-only sheath hardening method (though a much better, tougher steel was used to start with, greatly improving penetration ability), same block-TNT filler, similar (but much more reliable) delay-action base fuze, and the same unreliable Lyddite-filled one-piece booster can on the tip of the base fuze).
The Krupp divisions manufacturing naval armor and naval projectiles simply fixed things only when it was literally "hit on the head" by a problem, such as the rather truthful British claim after WWI that British WWI-era KC-type armor and, it also turned out, the several U.S. WWI-era KC-type armors were better than Krupp's own WWI KC a/A armor--due to Krupp never testing its KC armor systematically with CAPPED AP projectiles, since it started making KC a/A armor prior to the introduction of capped projectiles and didn't want to have to redo its extensive test library (it didn't understand how different capped projectiles were and what had to be done to counter them in changing its too-brittle KC a/A armor manufacturing methods, which it never did until the British claim shamed it into doing so after WWI). This reactionary mentality ("if it was good enough for Grandpa, ..." and "we are already the best") seems to saturate Krupp work up through the end of WWII.
Coupled with the rather tiny German Navy and the fact that in many cases
the defects were never reported ("dead men tell no tales"), Krupp never
did much to improve any of its projectiles once they were first designed
and issued--for example, it would completely replace WWII German tank ammunition
that it made with improved types after extensive battle results made defects
or limitations obvious, but did not gradually improve the existing German
Army and Navy ammunition much, as U.S. and British designers did