https://vsbattles.fandom.com/wiki/Rheinmetall_120mm_Main_Gun
23.6 megajoules, 14400 square millimeters (120*120). So it's 1638.(8) joules per square millimeter.
That's equivalent to someone punching you with fist for 70472222.(2) Joules, or about 0.01684326534948 Tons of TNT. Or for full-body feat (1.73 square meters, 1730000 square millimeters), 40828000000000 Joules, 9.758126195029 Kilotons of TNT.
So you're not immune to cannons via full-body feats unless you're at least 7-C.
P.S. being punched by fist isn't a full-body feat. Full-body feat is like explosion or "you're hit by projectile/weapon bigger than your own body".
P.S. if character is really big or target is very small, good portion of AP is simply wasted as it can't connect with enemy's body.
E.G. If your body's surface is 2 square meter and you're kicked by giant whose limb has surface of 2000 square meters - then despite giant having Low 7-C AP of 1.1 Kilotons of TNT, only 1/1000th of that actually connects. That's since vast majority of giant's limb can't connect and strikes the ground instead - so only 1.1 Tons of TNT, or 8-C+, actually reaches the target.
For another example: if you shoot a death beam with surface of 2 square meters at target with surface of 0.5 square meters, then only 1/4th of AP connects. The rest misses and hits something behind the target.
Also: should we ever adopt the new system of AP/Durability that takes Piercing into account? One that considers "energy per surface area", rather than just raw energy. Transition to that system should be slow and methodical, with old and new system co-existing for indeterminate time.
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