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Projectiles that do piercing damage do not scale their piercing damage with technology improvements. In vanilla, this means that basic tank shells are physically incapable of piercing two Big Biters regardless of how many technologies you've researched. This is unintuitive because each level of the infinite "Projectile Damage" technology boosts the damage by 100%. It feels like doubling damage output should also allow it to pierce twice as many things as before.

Here's an old thread talking about it

I noticed this when making a mod that adds a new piercing style projectile that's available much earlier in the game. But after writing this up it feels like a bug with the base game.

Can you not simply increase piercing power in technologies with the existing API?

It's impossible to do using technologies (as far as I can tell). "ammo-damage" modifiers don't apply the upgrade as evidenced by the screenshot below. I only used Creative Mod to make it easy to toggle techs on and off. It looks like the runtime API mirrors the technology so I doubt it would work.
I noticed this when trying to add an early game ammo that does piercing damage. It's a lot more obvious that piercing doesn't scale when you're killing low HP biters. If you increase the piercing power to deal with the low ceiling then the gun can kill entire mobs of small biters with a single bullet. If you keep it low then it only kills the first small biter and might as well not even have piercing mechanic.