Technically, there is a
9-A durability feat involving the Mk 4 suit.
Cal-141 only had a Mk 4, and was surprisingly the only Spartan to have ever survived a direct hit from a gravity hammer to the head; albeit only briefly, similar to Johnson surviving Guilty Spark's laser. But other instances portray gravity hammers as being capable of flattening multiple Spartans. Jerome survived Atriax, flicking him off, but that's not really a direct hit, and not an indicator of being higher than
9-A
Also, the
8-C durability feat initially came from Fuel Rod cannon; which Flo and FanofRPGs initially thought it was 1.5 Gigajoules, but I couldn't find a source. But I found a 3 Gigajoule calc that used outdated formulas. Additionally, the Spartan being vaporized to get such a calc was surprisingly a midget; which also means the suit she wears would have to be much smaller compared to other Spartan suits. Anyway,
the calc on my blog gives the details. the results ranges from 262 Megajoules to 1 Gigajoule, depending of Titanium was melted or vaporized. And the high end is just a hair below
8-C funny enough. Additionally, the keyword is that he can barely survive it, meaning it's unknown if he actually scales to it fully, but even a reasonable low-ball would still make him pretty high up in the
9-A department. And actually, it is the Mk 6 MJOLNIR that can just barely survive a Fuel Rod Cannon.
So yeah, considering Cal was able to briefly survive a headshot from a gravity hammer, and once repelled a blow from a gravity hammer, she's widely regarded as being physically the strongest Spartan. And all of that is despite Mk 4. And if this Mk 4 Spartan is capable of better physical feats than even Mk 7 (Gen 2 is actually Mk 7) can do, there's no need for separate keys for different MJOLNIR's as they're all consistently
9-A. Just suitless and with MJOLNIR are the only necessary distinctions.
Also, the energy shields as mentioned are more designed to repel energy attacks. Plasma weapons heat through the Mk 4 suits like a hot knife through butter, so that's why the UNSC copied the Covenant technology to protect against said attacks. The Mk 4 suits already had good enough protection against solid projectiles and melee attacks, so they don't focus too much on upgrading that aspect. Mk 5 is said to give up to 15 Megajoule protection against plasma weapons and that Mk 6 is twice is that, but that's
Wall level+ to
Small Building level respectively. But we got better
9-A feats, so we can stick with that.
