Have you ever used plated bullets before?
I use Berry 158gr .357 double struck for my standard IDPA/SSR load, and they are accurate and reliable.
I use 4.3gr HP38 to make power factor, and the "crimp" is just enough to straighten out the bell.
If you crimp a plated bulet very much, you'll break the plating and get patterns instead of groups.
Why are you using an ultralight load with such low speed with a SWC? It may not be sealing.
Did you clean ALL the lead out of the barrel before shooting the copper bullets?
Revolver bullets are always seated to cannelure if it has one.

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Science plus Art