I kind of had to fudge as I don't know the "scale" the 3.0 will be, I know its going to be between 1/50 and 1/60 (yep, there is a good possibility its going to be bigger than the Yamato)
I designed the Muzzle cap and the diameter of the bore based on a 1/50th scale of 16" (about 406mm), making the bore .32" (about .228 for the v2.0). Its going to be intricate to put together (its very very tiny). But I was able to dry-assemble one out of plain paper, my pocket knife, and a piece of tape. It looked awesome! Wish I had it at home to take a pic of it.