Go at the slightly rusted barrel with some JB bore paste (per instructions), then clean completely
using both oil-based solvent and final soap/hot water.
Thoroughly oil the bore down w/ a couple of patches of BreakFree CLP (or
WeaponShield) then oil it down again the next morning.
You're done -- and should not have a problem again as long as final step is BreakFree or its equivalent.
Forget "bore butter."
(!!!)
All it's comprised of is bees wax, olive oil, yellow food color and a pinch of Wintergreen for smell.