... how do you do the two fillets?
I assume you mean the rimbases? There are three different curved surfaces on the rimbase. The easiest one mates with the trunnion, a plain cylinder. The other two mate with the breech; the upper portion is part of a cone and the lower portion is part of a sphere. The easiest way to deal with it is to machine a flat on the barrel and the curve for the trunnnion, attach it somehow (welding, epoxy, etc.), and then fill in the void between the rimbase and the barrel. Apply paint as necessary.