I've discussed this with a number of folks at Holland's over the years, Winks, Wilkin and others, and can confirm that. Don't know the fellow from Westley. I've also discussed it with quite a few others in the London trade and have gotten the same answer.

9.3, I would highly recommend anyone interested in this issue to get a copy of the new (third) edition of Graeme's book, and read Chapter 13 carefully. It's the best discussion I've seen of DR barrel damage issues anywhere. Graeme is a real sharp guy, a DR nut, and knows everyone in the trade worth knowing. He covers some of his discussions with Holland's about OSR. For anyone with a double rifle, I consider it required reading.

I have easy access to several doubles with OSR. I'm going to TRY to photograph it. I'm doubtful.

As for a test barrel min wall thickness (I'll admit I don't see much need, as there are lots of rifles out there with OSR), I'd suggest a min wall of the thinnest in new guns. The min wall in current Chapuis 9.3X74R doubles (I've measured a number) runs around .078".