I have a Green Mountain .45 in a T/C Hawken stock, 1/48. Shoots PRB's really, really well. Rifling is probably deeper than what you are looking at.
I shoot a .440, .018 pillow ticking patch and 50 grains Grafs 3F. That was the sweet spot. It did not do well with .015 patches, .440 or .445's.
Shallow rifling on yours, I would be inclined to start with a thin patch and work up in thickness if needed.
