As said above, it all depends on the thickness of the barrel wall, at the muzzle. Teague will do steel or damascus but only if there is plenty of wall thickness. I don't think they differentiate between fixed or thin-wall interchangeables.
They recently refused a set of Whitworth steel barrels on their wall thickness that I sentr them.
Another issue raised by Demonwolf444 is the matter of reproof. Teague will not do any insert choke work without submitting for reproof (as is correct by proof rules) and then you enter a whole world of jointing, bore, chamber, rim recess spec which could make the job much more challenging!
