Mike's point is a good one. Barrels typically aren't concentric, and it takes a thorough check to make sure there's not a thin spot somewhere. But if .025 is as thin as it gets and if the bores aren't pitted (often an issue with Damascus), it should be safe.
As for the proof issue . . . a gun could be in proof from what's stamped on the flats, and still have minimum wall thickness of less than .025. On the other hand, barrels might have been honed enough that the gun is now out of proof because it's more than .010 over spec, but it could still have .030 minimum wall thickness, if it started out with thicker walls. Personally, I'd pick the latter gun over the former, but I'd probably try to negotiate a better price based on it being out of proof.
