A bit more research suggests that due to the burning characteristics of the slow burning powder the charge should be increased rather than decreased.

Next time I visit the range I will increase the load incrementally from 5 to 6 and 6.5 grns of Herco and see what happens. I have also sourced some ball of .460 diameter and some slightly faster powder (Unique), so still some options to pursue.

I will update this thread with any progress after trying the above alternatives.

Regards
Brian