I might be misinterpreting what's happening here, but my understanding is the little gray bar under the DC health here is the firing cooldown.
I apologize this gif is probably nearly unviewable.
Essentially, sometimes when the cooldown finishes but the deck cannon isn't reloaded yet, when it does get reloaded, the cooldown bar runs again before it fires. At least that what I think is happening. It definitely doesn't fire before the second cooldown runs through.
Is that the intentional behavior? I would think it would just shoot as soon as it's reloaded if the cooldown had already passed.