For the British guns, the coverage of proof in Volume II of Brown's "British Gunmakers" is the most accurate and coherently presented I've found. Sadly, like virtually all other reference works on British proof, it's shotgun centric, and of only some help with rifles. I use Wirnsberger too, but his coverage of rifles is poor as well and, again, contains errors, although not as many as most works do.

What is needed for this site (dedicated to double rifles) is an accurate and coherently presented treatise on British proof of rifles under the rules of 1875, 1887, 1904, 1925, 1954, etc., as well as the common loads usually marked for each caliber for proof purposes. Again, I know of nothing like that that's been published.