.450" MH: a man after my own heart!
It sounds like a little bit of excess headspace (can check on a chambered case with feeler gauges; I don't know what the correct max headspace should be, but I imagine in a 130ish yo military rifle a bit of slop is acceptable) combined with the low pressure of BP. The lower pressure may mean that the case head is not forced back hard against the breechblock, meaning the primer can back out a little into the space. I've seen it, but never regarded it as a problem.
I have used 30gr ADI AR2205 under a 500gr gg bullet. It was accurate, mild on the shoulder, but I'm not sure I liked what may have been happening inside pressure wise - very small charges inside cavernous cases worry me. Trail Boss could be a winner, but I've not kept up with that development. 1f *real* gunpowder works very, very well for me!
Have you consulted the Martini gurus on:
http://britishmilitariaforums.yuku.com/RG
