Usually when that happens,,
It's an indication that the gun was fired with a little storage or protective oil left in the breech.
It happens,, even a little oil with the heat of burning powder turns the fouling into a hard almost tar like substance.(Bore Butter will do it too!)
The best way I have found to get that out is the pump method with a soup can and a little gasoline, sure people grouse about using gas but you just need enough to flush in/out through the breech not the entire barrel like a water wash.
Just be outside and use care, follow with brake cleaner then soapy water like normal.
I've never used/needed a brush in the breech hole in my life.
If you use oil for storage, you have to use an alcohol cleaning/flush before shooting, no traces of oil.