Hello,
As it's a Ruger Old Army I wouldn't want to use a ball of smaller diameter than the recommended .457
I have measured phials of 4, 4.5 and 5 grns of Herco and will try these next to time I go to the range. The original 4.5 grn load is fine when it works, but I'm not happy with the occasional light discharge and the evidence of unburnt powder which accumulates in front of the muzzzle.
If this doesn't work, I will try a slightly faster powder such a Unique.
Brian