I have two SMLE's in .410, one original 2 1/2", one 3". We fire lead balls out of both. Is the barrel in your Siamese toast? I know the term "8mm" is rather a loose one. What is a Siamese bore? If a 7.62x54R works as far as brass goes, would it be feasible to blow out the brass to the bore size and fire custom lead bullets or balls out of it. Maybe I should quit while I'm ahead!