Bill, I think that you're going to have a difficult time getting the Gibbs .461 No. 2 in the Martini action. The No. 2 is a pretty long cartridge and I don't think it will make the corner. Any falling block action should work well though.
In the 1870's, Gibbs wrote Halford that he was using .460 caliber barrels at that time and I believe by then, he was using Metford rifling. There is a picture in Kirton's book with a mould for the Gibbs bullet. It is marked .461 Gibbs, 570 grains,
.453 diameter. Depending on what thickness of paper was used, it was probably wrapped up to .460" or .461". The Europeans tended to wrap there bullets to groove diameter while the U.S. shooters wrapped to bore.
