Question about shield overkill.

Projectile weapons will destroy the shield, and then continue their travel, but at a slower pace (depending on how much of their power they need to pierce the shield). The slowed projectiles will also do less damage - in your example, the bullet would do 1/4 of the damage, because it already used 3/4 of its power to pierce the shields, so it would do 0,25*70=17,5 (no idea about the decimals) hull damage.

This can be easily observed when using Mega Plasma Cannons.