Uh, well, not so much rare as non-existent!
The Peabody-Martinis were built for Turkey in the 1870s, in a chambering described as 11.43x55R. Another source says 11.43x41R. Quite possibly the cartridge was originally 41mm long, and later increased in length. In any case, after such a long time you would need to check the chamber before trying anything.
But one thing is sure: the Peabody Martini was a black-powder rifle. 7.62.x54R was the cartridge introduced with the Mosin-Nagant of 1891, as a smokeless cartridge. So please forgive me if I ask you: how do you know it is chambered in 7.62.x54R? An original Peabody barrel cannot be chambered to this size - it would have to have a liner fitted. Or maybe someone got really creative with a Mosin-Nagant barrel? Or fitted an Egyptian 303 Martini-Enfield barrel?
Please check really carefully what you actually have. Nothing is impossible when Bubba's around, but this sounds improbable.
Patrick