470NEBD:
Remember that in almost all cases with correctly-designed bullets (hard-cast, paper-patched or thin jacket), the Nitro-for-Black load is understood to generate less chamber pressure that the original BP load. ...and with heavier bullets!
You may well be doing any BP rifle a big favor by feeding it with an appropriate light nitro load.
Read Wrighty's book "Shooting the British Double Rifle" for the inside skinny on this.

In my experience with a number of vintage doubles, the rifling twist-rate is never a consideration.

Hope this helps