I just had a realization..... IS core evolve like an organism.
The more you expose it to certain elements....
It will slowly adapt to the elements...
The reason why Byakushiki is lower Shield energy than normal IS could be because the IS core did not take part in IS competition as much as other IS....
Also may be due to the fact that the battle that it take part in: the weapons are not powerful enough to even scratch it. Since there is no threat, The Core 01 believed that there is no need to increase shield energy....beyond what is needed.
Which leads to:
Gray Scale may actually be a more powerful when it is first introduced (probably early second gen), may have served as Barrier Void Weaponry during that generation.
But over time, other IS begin to develop countermeasure/stronger resistance to it.... which is why in spite of being shield pierce weapon, it did not force activation of Absolute barrier.
Which probably why France is exerting pressure on Dunois because they do not know exactly how long will it take before other IS cores finally gain immunity to Gray Scale.
Which also explain why Byakushiki has Yukihira mk2. The Energy field generated by it is something new which other IS has to yet to learn how to cope with...hence maintaining the effectiveness of the barrier void.
Which also explain why other countries are pushing the development of third gen weapons when second gen weapon work just fine.
Because Second Gen IS weaponry are basically the same (Soup-up version of conventional weapon remember? Repeated exposure to same type of weapon will allow the IS to learn how to cope with it eventually (First sign: Increased Shield energy, allowing it to last longer under machine gun fire))
Reason why third gen IS weapon is developed because the unique features of each third Gen IS prevent such thing from happening.... Adapting to one 3rd gen weapon will leave the IS weak to another 3rd gen weapon....Everything kind of balance out eventually....as the IS goes into circle trying to figure out the best way to increase the effectiveness of its shield.
Question: Why didn't other IS develop counter-measure against Chifuyu's unit?
They did: Ranged weaponry...
Admittedly, it is not easily to stop a weapon that can bypass Energy defenses and they are still in the midst of trial and error.
Edit: Why Byakushiki 2nd shift has no increase in Shield energy? It has double ignition Boost.... theoretically: no IS can hit it and since no IS can hit it, there is sufficient energy in Byakushiki to take down other IS in one strike.