Here's an update on my progress. First, a big Thank You to
@kenbro who has put me in contact with a friend in the States who is trying to do something quite similar. His friend has been at this for three years now while trying to figure this out and has helped me immensely.
What I need to do is first apply for Type 03 Federal Firearms License (Curios and Relics) as you must have an FFL to import any guns made after 1898 and this license allows me to import guns that are at least 50 years old and considered collectible. Sporting shotguns can be imported with this license and it is the cheapest and easiest non- commercial FFL to acquire.
Next, I need to buy a shotgun or at least know all the particulars of the gun I plan to buy including overall length, barrel length, maker, country of make, serial number, caliber/gauge/chambering and any other identifiers. I can then use this information to obtain an ATF form 6 (import permit) which can take 3-6 weeks for approval.
When I have an approved Form 6, then I need to send it to the appropriate authority in the UK in order to obtain a UK export permit surely adding a few more weeks.
With the Type 03 FFL, the ATF Form 6 and the UK export permit in hand I can then fly from the UK with the gun assuming that I have a temporary shotgun permit which allows me possession abroad (unless the export permit covers that).
Then I need to find an airline that will allow firearm transport and a connecting airport from Inverness that allows the same. Usually I've flown BA through Heathrow because options are limited to H&I in Inverness. Paperwork in hand, a secondary customs visit in Denver should have me headed home
I could skip all this and use a shipper in the UK plus a customs broker in the states for about $1000 for just one gun. Savings are to be had if shipping more than one gun. Downside to this is that a US licensed broker/ importer has to engrave my new gun with his ATF approved identifying marks thus ruining it.
Its all quite simple really
Scott