I have a sudden idea: instead of the current damage reducing model the carronade has, give it the nominal beam damage reduction profile, with a catch:
On top of normal damage reduction, the carronade impact profile will get wider and wider, and as a result, shallower.
If this gets to the point of the damage depth being less than one, the shot will be counted as doing 0 damage.
Any width in excess of 80% of the cross section of the ship's armor, rounded simply "misses" the excess.
Ships with shields active count as having a cross section 20% larger for consideration of carronade damage missing.
Basically, making carronades the weapon of choice for evaporated shields by dumping a large amount of damage, while also essentially reducing the armor level of whatever it hits by 1 over an area (assuming the carronade is doing significant damage to it). This makes it a lot easier to jack up the base damage of the carronade a bit, designwise, without making it deal massive penetrating damage.
I can also imagine a really silly situation where a carronade is fired at a 1 armor civilian design from far away, and the result being the ship being stripped naked of it's armor, but surviving for the most part, aside from some potential shock damage (though, whether a 1 deep crater should shock damage or not is a good question to ask).