Somebody had suggested it might be a 577 round any thoughts.
In terms of original proof, no, it couldn't be. As I said in my previous post, both barrels show London marks in use until 1887, and are for a .500 rifle. From your photograph of the flats, starting from the breech end (reading right to left), the marks are:
London provisional proof mark (lion rampant over the letters GP interlaced in a cypher)
London view mark (Crown over V)
Bore diameter (39)
London definitive proof mark (Crown over the letters GP interlaced in a cypher)
Nitro proof was still in the future when these marks were in use.
Prior to 1887, the British determined and marked bore diameter of rifles in exactly the same way as they did smoothbores - in gauge. In other words, a 39 gauge ball (.492") would drop through the bores of your gun, but a 38 gauge (.497") would not, so it's marked "39" gauge. Due to the method used to determine gauge, it's important to understand that this mark represents, with respect to rifles, LAND diameter, not groove, which would be 10 thou or more greater. The various British .500 rifle cartridges from the period used bullets of .500 to .510". Prior to 1887, the rifles chambered for them were marked "39", a few "38". Understand that this mark addresses only bore diameter, and tells you nothing of the rest of the cartridge (British proof marks of the time didn't). There were a number of .500s of different lengths, including several based on bottlenecked .577 cases. Only a chamber cast can tell you which case this one used.
Pre-1887, the various .577 bore rifles were marked "25".
Note that the marks on both barrels are the same. If this piece were built as a cape gun in, say, .500 BPE/16 gauge, the rifled barrel would be marked "39", and the smooth barrel would be marked "16". Unless there was some strange 39 bore shotshell at the time that I've somehow missed, the proof marks say that this piece was built as a .500 double rifle that has since had it's right barrel converted to a shotgun without re-proof. Unfortunately, such conversion would hammer the value of the piece pretty hard.
