Shooting lead without a gas check? 

At higher velocities it was likely some of the lead bullets without gas checks were leaving significant lead residue in my barrel, thus accuracy suffered over time.

Time to clean the barrel. Can you say dirty?

It is also likely that without a copper gas check, I learned, the heat from the shot perhaps melted some lead at the bullet base. 

Cleaning the barrel with solvent and patch, I learned, was not an option as the patch never came out clean. 

I had to use a solvent and brush with some JB Non-Embedding Bore cleaner.

Wear rubber gloves to clean your barrel when using lead. 

Lesson: Clean your barrel wisely and watch the faster lead bullet velocities without a copper gas check.