400NE

You could get a new set of barrels in Austria or Belgium for around $7000. Proofed there or sent to England, which would be my choice.

I think you are right about sleeving the barrels for a smaller caliber. I would go even smaller, to a 303 or such.

I don't think any Proof House would deal with sleeved barrels though. I'm referring to rebore and sleeving the existing barrels, not putting new ones on a mono bloc.

It's a mess and the options are all quite expensive.