Help with fire change i want to make, and other ideas

Hello. First of all, I want to make it so that when a wooden block is on fire, instead of it just "dissapearing" like in vanilla, I want it to get replaced by "charred wood" or "charred planks" which burns slower. How can I go about doing this?

Second of all, I created a gun and I would like the bullet to lose damage overtime while travelling in air. Is there a way I can do this? I already tried both but I am new to making mods.

Thanks!