Fighting a giant, armored, magical monster.
Why is your first thought not "I need a shaped charge explosive"...
Nah, gonna do regular pew-pew. Huh, didn't even scratch its armored head; must mean it's harder than copper... but by how much... and who knows what the internal structure is like.
Shit, my buddies are gonna die! Let's try 7.62 NATO AP rounds, even though it's a giant, magical monster, whose head armor looks like it's several inches thick... Should be fine! Let's ignore the fact that small arms AP rounds can only go through a couple of inches of steel at most.
The best Level IV ceramic composite armors get these rounds into the "do I have enough muzzle velocity to penetrate" territory; one of the qualifiers for level IV is stopping 30 caliber M2 AP rounds.
It's a giant, magical monster! Stop trying half-measures!
I hope the webnovel explained this in a way that makes more sense, or maybe the next chapter will show something unexpected, like those rounds not actually penetrating... I don't have high hopes for that though...