I have a character in my campaign who has spent a ton of work building up as firearms character, but their dex is low. A big part of this characters arc is using their intelligence to accomplish things that would normally come by strength or other physical means. To that end he is currently investing all his downtime efforts into a "Smart Sight" system that would allow him to aim his ranged weapons based on INT. 

Im more than happy to let this happen, but I can't figure out how to do the home-brew options to make his ranged weapons base their hit and damage modifiers off of INT instead of DEX. By this I mean that I can't get DNDBeyond's code to recognize the change and build it into the ranged weapon calculations. I have been trying to do it using a feat, but would this require building a custom sub-class for them?

Thanks in advance. 

What you want is impossible, the player will just have to “Customize” their ranged weapons to reflect Attack and Damage bonuses as if they were using Int instead of Dex.

PS- Nice Avatar. 👍

Best option may be to build the weapon as a custom cantrip instead.