JB101
Those that I have seen are very (or can be) very accurate rifles.
The bullet is 'it' with all BP (ML & BPCR) rifles. Pure lead or pretty soft alloy (say 30:1 lead/tin) Get the right mold and pretty much straight from the mold - after lubing.
Over powder wad either from gasket material or HDPE .060" thick is a good start.
Pick a starting point re. powder and work up - Swiss powders are very good their version of 2F (#3) wqould be my suggestion
Here is a link to some similar work on same cal. http://blog.davide-pedersoli.com/dev...i-gibbs-rifle/
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