I've found that just pure DPS its 30% more efficient to just put 4 standard cannons than this, but we all know big cannons arent very good and so its still better than the big cannon
note: in my comparison i tested sole-y based on width. so i tested 1 capital cannon vs 3 big cannon vs 4 standard cannons
this cannon is much larger and harder to protect, is much more expensive than just 3 big cannons or 4 standard cannons and is still not powerful enough. I do understand that is severely outranges every other gun, but I feel that maybe if range was decreased, this could be more balanced.
Also, what is the reason you want 4 barrels? FASTER FIRERATE!!! This thing has a really slow firerate and to make up for it you have to make it deal high damage and high explosion radius which you were tinkering with a lot before. If you increased firerate, you could still be able tolower damage even more without it becoming underpowered. I do understand that you have like a dozen people that want a more powerful gun, but because this gun is so big and difficult to use, it should have some upsides to using it and right now it does not.
I also know that you don't want to just make another overpowered cannon mod like every other one out there, but this is almost sad how weak it is, and I think you can bypass a few balancing problems that you were hving by increasing firerate.