The piece in the picture is on a metal display carriage. Check to see how far the rifling extends into the bore. Many sleeves for modern cannon only have rifling near the muzzle. It is a safety precaution. When #1 sponges the bore after a shot, #3 plugs the vent with their thumb. That creates a vacuum when the sponge is withdrawn. Done properly, there is a full bodied pop-gun Boomph! as the sponge leaves the muzzle. The vacuum starves any lingering embers & prevents a premature detonation. Needless to say, a smooth surface is less likely to harbor debris that could cause an accident. It is also much cheaper to rifle 12" of the bore. In some display pieces, the bore only extends about 18" for the same money saving reason.