It sounds like a good idea, but I don't believe Green Mountain offers such a barrel. At one time they cataloged a .458 barrel an inch across the flats with a 1/20 twist for 45-70 rifles. One of these could be milled down to 15/16. One problem with this though would be getting a mould the correct size. Someone out there could probably make you a custom mould.

W.E. Rayl offers custom made barrels . He may be able to help you. His Phone# 304-364-8269