YouTD

The alternate functionality of this tower is neat, values just need to adjust for the power of each effect based on how the game is typically played.

Hullbreaker is most powerful against a high armour target. For example, max level, full stacks, it will ignore 60 armour against a 200 armour target and 30 armour against a 100 armour target. As armour reduction is often a priority for players, enemies will trend lower and the value of the ability then has to measure up against the power of Shrapnel, which at a glance gives up to 60% bonus damage to your main target. Which is very, <i>very</i> incorrect.

Overdrive, at 30 stacks, gives up to 150% attack damage at level 0, and 225% at level 25, which is then converted 20-60% into damage, for a total additional 30-135% damage on the primary target.
Just to reiterate so I'm clear, at level 25 the tower gains up to 225% damage from Overdrive, then an additional 135% damage to it's primary target from Shrapnel. (or 150% + 30% at level 0).

I would recommend a nerf to shrapnels scaling, so at the extremes its less powerful. Additionally, to really juice the Hullbreaker, I'd give it another effect. Even if the numbers are aligned in a way to roughly be balanced, Shrapnel is more appealing due to its universal effectiveness.
I hate leaving without a suggestion so I'll offer my one stupid idea; every 10 seconds the Hullbreaker Rounds are active, the next attack stuns for 1 second. I have some justification but I feel like I've talked too much already.

Correct me if I'm wrong, but did you write that Shrapnel deals 60% bonus damage at lvl 25 against the main target, like 160% total? Because my intention and the way it's coded is that it only deals a fraction of its total damage (20% - 60%) against both main and secondary targets.