To me a piercing+explosive weapon would be too simple, field two of those and you have a ship killer.
Even as common as the gat flack combo is, it requires timing, teamwork and resisting the urge to shout rude words at the guy who missed his flak shots
while the piercing+ explosive weapon sticks too much simplicity and does not encourage teamwork.
I reckon a piercing+fire gun would be nice, even if it doesn't make sense on how it would work.
BUT, you have to ignore the pretty effects and consider the meta and what the player will think to this.
As one player said, we will have gat flack combos on galleons if you introduce a heavy piercing weapon. I believe that it may reduce the supportive role of the galleon and turn it into a killing machine. Right now the galleon is more or less okay, but if you introduce a way for the galleon to very quickly kill judging by the damage of the current medium weapons, the meta may change to a two galleon combo where one galleon can help out the other.
because of the damage, most other ships will not be able to compete as you can secure each other behind and other blind spots.