When restoring cast iron machine tools, voids in the casting are frequently found to be filled with some kind of putty so the casting looks nice. It kind of comes with the process. If you read Muller, you will find him talking about this issue. So I would expect any cast iron barrels, except maybe the most premium, to have repaired casting flaws. They are probably not a cause for concern unless in the bore or extremely large, and a reputable foundry shouldn't be releasing anything with flaws that are remotely hazardous.
The Hern barrel is underbored; 2.25" versus full 12 pounder diameter of 4.62". This is most likely a liability issue to prevent lawsuits or at least have a leg to stand on if someone sues them claiming a flawed product. So you should verify with your reenactment groups whether underbored guns are acceptable. Shouldn't matter if all you are doing is making smoke but sometimes groups like that get caught up in stupid rules.
The originals were brass/bronze. I think South Bend Replicas offers a brass one, probably underbored also, but it is not going to be cheap. Try
their website for a contact number. They mention the mountain howitzer but there are no photos on the site.
