... cast their barrel with bore already hollowed out , for lack of a more technical term ...
The technical term is "cored" or "with a core", the core being the part of the mould that creates the cavity. Casting a short barreled mortar is quite different from casting a long barreled cannon. Because the mortar is so short and its bore is large with respect to its length, the core can be made quite robust and it will not deflect when the casting is poured. Because the walls are relatively thinner compared to a solid casting, the shrinkage problem is lessened because the metal solidifies more quickly, so you shouldn't get shrinkage defects the way you do at the center of a solid casting.
Modern casting methods allow casting to near net size so casting way oversize and machining to final size is a choice, not necessarily the only way. Since you name no names in your post, I would say that manufacturer reputation is more important than manufacturing details.
Looking at the pictures of the larger cast iron mortars makes me think that they generally were not machined overall on the outside. I would think that only the trunnions, the bore and the vents were machined with the rest left as cast. Most of the 13" mortars appear to have a muzzle band but upon close examination, the band appears to be a narrow strip that was machined, probably so the barrel could be held for boring, and the rest of the outside is "as cast."
