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Dear Mark,
What a beautiful double you have acquired! As an engraver I am greatly impressed with your lockplates, and can assure you that the incredible buttplate also took an enormous amount of time.
As for dating your BP rifle, we are in luck for two reasons: first, on the BB threads you referred me to several people saved me some looking by stating the .577 Snider came out in 1865-66. But most importantly, you posted clear pictures and if you look closely you will see the number 31 on the breech side of the provisional proof mark (rampant lion on bar, with script-letter GP below it). This is visible on the right barrel, and just barely visible on the left, suggesting the barrels were struck at some time - perhaps simply before the definitive proof.
Now if you can get your hands on the 1977 Gun Digest there is a superb article on British proof and proof marks (pgs. 161-72). Therein we find that the rampant lion provisional proof mark came in with the 1855 proof law, and that the stamping of bore size was done at BOTH the provisional and definitive proofs. This would explain the 31 beside your provisional proof marks, and the 26 beside your definitive proof marks. When the 1868 proof law came in, the provisional proof no longer had bore size stamping. So there you have it, your barrels were proofed between 1855 and 1868. Allowing for manufacture into a completed and engraved gun, I would say 1871 is about the latest it would have been made. The lack of rebounding locks also points to an early date, as does the .577 Snider cartridge to some extent. On a gun of this high a quality, the lack of some kind of foreend attachment other than a wedge also points to late 1860s or early 1870s too, it seems to me. Some guns "look" earlier, and some later, so by using the "forensic gunsmithing" information we have, I think a date of 1866 to 1871 is in order.
For other information on your double rifle, I would suggest you try side-light and a 10X loupe to go over your barrels some more. but more likely the loading information was in the case - now lost.
I would think you can get your bored-out barrel lined, and suggest you contact Kirk Merrington in Texas. He is English and English-trained and should be able to point you in the right direction if he does not do this himself. Phone is (830) 367-2937. JJ at Champlin Arms in Enid, Oklahoma is another person who could refer you - believe somebody on the BB mentioned a source JJ uses.
It would be nice to have your double coverted to .500 X 3" or the like, but the cost would probably be prohibitive, and would require expensive re-regulating, assuming it is strong enough for such a BPE cartridge. Keep the re-regulating in mind if you change to any other cartridge. And BTW, be sure to Cerrosafe your chamber/bore to see the exact cartridge and bore size, if you have simply been told it is .577 Snider. It may be something else! There were a wealth of cartridges tried in the late 1860s and early 1870s as the BP Express rounds were born and settled on.
You will find an article which includes loading for the .577 Snider double rifle in the Autumn, 1994 Double Gun Journal by Mel Swanson (pgs. 117-121, "Loading English Black Powder Cartridges" Part II). You are welcome to get back to me later about loading with BP. I am not experienced with nitro-for-black loads but can refer you to those who are.
Please post this on the two BBs so others can see it. It may cause dissenting remarks or other information to appear, and either are always good.
Aloha, Ka'imiloa

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