Whilst Patrick Chadwick is resting in his far away land, I would suggest reading a previous thread of his that relates fairly closely to your future reloading endeavours for this rifle. Whilst the actual cartridge and rifle are a little differernt, the theory is the same. (In fact, the nature of the lock up in a rolling block is probably MORE suseptable to "off-axis loading" induced barrel whip but in a more vertical plane than the M1871.)
So, to jump ahead a bit, and to hopefully save Patrick from having to re-invent the procedure again, here's the thread:
https://www.milsurps.com/showthread.php?t=4777
"To carry on where we left off...Smokeless (powder) in a Mauser M1871 or 71/84..."