We know at Trafalgar Nelson had Victory's loaded like Super-Size-Me shotguns with buck-n-ball (a so-called "Duplex Load," where we load buckshot and then a slug into the same shell so the first big penetrator helps soften the target for the smaller ones following)... Double Grape might need to be a Captain Special Ability, but I could see Buck & Ball as a one-time Opening Salvo for everybody, or requiring a certain number of phases without any firing on that side before you can do it again.

I think that previously-linked thread had my discussion about how Ares carronade rules extrapolated to all-carronade loadouts would make HMS Egmont a floating meatgrinder... I don't remember if they did that to a Temeraire too, or if that was just another trial of "Top to Bottom 24's."