TH44,

I have a friend with a Snider double & the bore is MUCH tighter than a Snider actioned rifle & with a better designed throat and also a much faster twist. His shoots BP very well & accurately.

I suggest that the loads on the British Militaria forum are suitable for a Snider Actioned rifle and should only be used as a very rough guide. Pressures will be a bit different in the tighter bored double.
It would be worth asking in the reloading section of that forum though, "The Double D" might have some loading ideas.

A note about the 25 bore - that is the BORE size, not the groove diameter which you measured at .583". In this case a 25 bore size ball will pass through the barrel, but a 24 bore won't.