Why is the HZ12 alt fire so unintuitive and annoying?

Why the hell doesn't the gun have an alt fire that just quick fires both barrels? Or at least let us change fire to both barrels at the same time? The way it works now is so annoying, I love the shotgun but damn triggering bother barrels just isn't very responsive.

Why is the HZ12 alt fire so unintuitive and annoying?