It's not clear how the methods of gun construction would lead to less material being used in the construction of piston engines. The internal pressures inside a firing gun are at least a couple of orders of magnitude greater than those found in your everyday four banger. Certainly, the cost of producing such an engine would not be reduced.
Most cylinder blocks are cast with a rough cylinder shape in the mold. The surface of the cylinder is finished by drilling out to the design diameter. In older versions of motors constructed with aluminum blocks, iron sleeves were press-fit into the block to serve as cylinders, since the metallurgy of the time did not permit having aluminum cylinders which could stand up to the wear of the motion of the pistons and rings. I think newer aluminum alloys now allow the iron sleeves to be dispensed with.
Large guns are generally built up from smaller pieces since it is easier to control the manufacture of several smaller pieces than one large forging. Also, the rifled liners in large guns have to be replaced periodically due to wear created when a projectile is fired. Shrink fitting facilitates this replacement.