I have a coil brass round that I thought was Martini Henry or a 577 snider however the measurements are a mix of each. The round has a paper patch on the bullet, caliber above the patch cal .524 over the paper is .577 OLth 3.429 case Lth 2. 735 case Dia .605 top and . 659 on bottom drawn brass rim dia .744.
On the top of the bullet there is what appears to be the top of a round ball, non magnetic. The bottom of the case is magnetic