I bet if you hit it with some copper solvent and a rag it will come right off. My guess is that's where some solvent pooled when you pulled your bore snake through and that liquid evaporated off leaving the copper residue deposit.
As for the HV ammo causing too much heat in your barrel, that isn't going to happen. HV 22LR ammo is only running around 1600fps tops IIRC, and this is the same barrel steel Savage uses for all their centerfire rifles that often seeing bullet velocities of double that on every shot. Some even clock up into the 3900-4000fps range (32gr .204 Ruger ammo for example).
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