I don't, actually. I just don't want a dual weapon unnecessarily in the game when I can just use my mousewheel for two weapons which are easier to get and accomplish the same thing.(Plus, I love the onyx blaster, sky fracture, fetid baghnakhs, dart rifle, queen spider staff...) This is a problem the Sky Dragon's Fury doesn't have, because both of it's functions are at least somewhat unique. I do suppose I am being pretty picky at the end of the day though. This gun isn't that unique in my eyes because for some reason OP stated that the cannonballs don't have an arc, and that just makes it a discount rocket launcher with low damage for me. I'm kind of wary of dual weapons anyway, because I have smart cursor turned on when I'm not building.
It is a pre-mech vortex beater, and it was quickly made, so naturally it's not perfected.
Why? Giving a projectile with piercing to a rapid fire weapon is kind of a big nono. And it seems to me like you're trying to hijack the suggestion.
The OP states that the cannonballs are fired at a slower rate, so making them pierce would both mimic the Cannon's abilities, and make them do something beyond just adding damage.
They would subtract damage, because giving a piercing projectile to a rapid weapon like this one makes some of the projectiles get nullified. I don't know if you're aware, but hitting an enemy with a piercing projectile gives them a small amount of I-frames...
It only affects other piercing attacks., so the bullets wouldn't be affected unless piercing ammo was used. The cannonballs are shot more slowly, so it can be assumed that their fire rate would be low enough for their damage output to not suffer.