Fraser, here is a little post on my Hughes Farquharson.
I plan on trying some H4198 in this gun soon with the 350gr Berry's bullets. My guess is starting at about 42grs and working up will find the sweet spot. I can measure the bore for you as I had not bothered to do that before simply because the soft .458" bullets worked so well. Also, I just use pin gauges to measure the bores in my bpe rifles since that is the measurement of importance when using black powder and a long match type bullet but these 350 grainers are short enough that I didn't run into chambering problems even with a dirty bore. A long PP bullet may be a different story.

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