I play with the .577"/.450" cartridge in Martini Henrys quite a bit. I'm yet to hear any of the people I regard as gurus on the cartridge and rifle express their complete satisfaction about the reliability and safety and/or accuracy of any smokeless load in same; that includes trailboss with a burning rate akin to some shotgun powders. As I recall, Bell took a not so subtle swipe at some of those people in his article.
Black powder for me in that cartridge, and that rifle, all the way.
RG