I would think you problem might be contouring but then you can simply strike up a lone tubes on a lathe.
Another snag may be the modern alloys used by many modern barrel manufacturers. We hit this snag using modern alloy sleeving tubes some years back which when welded gave a very different black finish to the original stubs. Only an aesthetic problem of course but it may be a consideration for your client.
The tubes used in the UK come from Italy mainly, there are a couple of companies doing them, one is part of the Beretta clan, Franco Beretta I think.
However, in the UK this has all become rather expensive and complicated by Brexit whereby stuff that was duty free before is now (sometimes) subject to duty.
Of course nothing has changed for the USA importing from Europe.
I suggest that if you go this route, you insist that the tubes are listed as 'Steel tube' on the commercial invoice, firearms not being mentioned at all. After all, until chambered that is exactly what they are.
This should make life much easier for both importer and exporter.
