Now if those were plasma cannons (not talking whatever the "carronade" is supposed to be), the plasma "bullets" might dissipate after the magnetic pocket that contains it wears out and that could explain decreased damage at range. Also explains why they have apparently unlimited ammo---figure gas-compression tech and good batteries could give that appearance despite that being as finite as solid projectiles.
A 'carronade' is a term from the age of sail. Due to a variety of factors warships were generally standardised on the concept of 8, 16, 24 and 32 pounder gun batteries. Frigates were the lightest of these ships, small, agile and generally equipped with a single deck of guns no heavier than 16 pounds. If they had another deck, it was probably 8 pounders.
This made these frigates fast ships and excellent for scouting, but not really suited to a battle line.
Ships of the line however carried batteries of big guns, often on 3 decks that were loaded from top to bottom with 16, 24 and 32 pounder guns. This made them very powerful in naval artillery, but slow and lumbering at best.
So what happens when you take a ship of the line and tear off the top gun deck?
Well, you get a heavy, slower frigate than normal that's
absurdly overgunned for its size with its batteries of 24 and 32 pounders, but with crap for range. These ships were called carronade frigates.
And that's kind of the role the Plasma Carronade has in game; a big weapon system that's rather close in and horrifyingly good at breaking other ships.