Nothing. Have you ever seen a 8mm Lebel case? Besides, PP brass is cheap and available. The only possible thing might be to re-bore to .348 Winchester, but that would leave the barrel rather thin. .348 brass can be used to make Lebel brass, but the results aren't always satisfactory. The smaller rim of the .348 case will often slip out of the tube mag, and jam the action badly. Also, they don't extract reliably from all rifles. The .348 case is about the only thing that comes close to the 8mm Lebel.