You are correct. But there is a supply of lathe turned 12ga 3.5" brass out in the world. That is only a bit easier to find than a heavy 12 ga blank.So why not just set back the savage or find a H&R heavy barrel single shot and convert that.
I don't think you can run pressures up much more than standard shotgun without converting brass to large rifle primer anyway. The weak link is still the brass.
He seems to be pretty reasonable about doing custom work as he did for my .338 barrel though it did take a bit longer delivery time over his standard work.Bartlein has replied and they nicely explained that they dont have the tooling for shotgun barrels.
I did hear back from Allen swan or the shop at least. They make barrels but I am waiting to see if they can make the diameter and length I would like.
Glad that you were able to get it sorted... For customs clearance you definitely would want someone knowledgeable. The other alternative is to find someone already doing import and see if they will roll your order into their shipment.
CALL OFF THE SEARCH! I have a barrel maker that is willing to make a 12 gauge slug barrel. 1.75"x30" 1-28 twist and material is 4140
Suzi has been very helpful.
Allen swan gunsmithing out of Australia was very helpful and willing , but customs on my end made it dubious at best that I would get the barrel. A local customs guy was very worried it would get through to me. Swan was confident, and they seemed very competent but I was able to find one closer to home.
I will update when the barrel is in hand. Thank you all for the help.