Andy52,
I have never seen a Hern Iron Works barrel in person. From reading their web description, I am a little skeptical because they cast the iron around a steel liner which invites a warped bore as the 2700 degree molten iron hits a room temperature piece of DOM tubing. Even if the bore liner is heated very hot, the center of the cast material (the weakest part of any casting) ends up between the liner outer wall and the outside dimension of the barrel. In the 16th, 17th, and 18th centuries, the strongest way to make a cannon barrel was to cast it solid and then drill the bore. That way the weakest part of the cast material was removed by drilling out the bore. In my opinion, the best way to put a steel liner in a cast iron barrel is to cast the barrel solid, drill out the bore, and then install the steel DOM liner with epoxy. Not that the Hern barrels are not structurally sound....It just that I personally like the more time honored way to produce a strong barrel even before it is lined. You might compare a South Bend Replica cannon barrel to a Hern Iron Works barrel just for your own assesment.
Also, if you do build a caronnade, I would suggest putting it on an appropriate sea carriage rather than a land carriage.....again just my personal preference. I hope this helps