Hugh:

I'd suggest getting in contact with Hornady. They have pressure barrels in .404 Jeffery, .416 Rigby, and .458 Lott. Ask for Mitch. They might be willing to test a few batches for you, but they won't be able to help with the .425.

As you're aware, the Birmingham Proof House also offers this service, and would be able to do so with the .425, but getting the stuff there might be a headache. Kynoch will also do this for you. They have state of the art equipment.

"Serious rifles have two barrels, everything else just burns gunpowder."