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I think more crimp is unlikely to solve the issue, lead projectiles are slippery and slide easily in the bore compared to jacketed ones so develop less pressure. Lead is also far softer than copper...

For cast bullets I'd suggest 4427 or ar2205 would be better choices, the unburnt powder isn't a safety issue although there have been historical reports of light loads of slow powders detonating it...

Not developing sufficent pressure/ barrel dwell time for complete combustion is probably the cause.