Then internal destruction Haki should also be ineffective, right?
Most likely?
I think his internals are less durable than his external body due to the fact that those attacks shouldn't be rendered completely useless but still powerful enough to not take severe damage from those attacks.
After all, Luffy already used internal destruction haki on Kaido with Red Roc - which only dealt superficial damage.
Honestly, I like the fact that Kaido's durability is not the only amazing thing about him. Hell, said dude hasn't even shown BASIC COA yet.
Kaido still has his hybrid and most likely his awakening which will be something much more amazing than his hybrid.
I do think the second round will be already over after 2 - 3 chapters tbh.
By the way, Yamato has ryuo and if Kaido has ryuo as well, this only makes him more threatening than before...
Gamma Knife is hax after all.
Correct, the hax to bypass the skin - it is similar to internal destruction haki after all.
However, the radiation which ruptures the organs is still AP based.
I don't think radio activity can be tanked by just having hard enough tissue.
My knowledge about radiation, especially gamma particles, isn't that highly advanced but is it not necessary how durable an object can be to withstand the particles - piercing at the body - as much as possible?
After all, there ARE robots which can endure some amount of exposure of radiation, way better than humans.