Since that's the business end of the cylinder it figures to get the worst of it.
There's probably as much powder and primer stuff as lead.
The lead is the easiest to avoid, use jacketed or plated bullets.
The other crud is going to be there regardless.
If you still prefer using lead bullets, a more agressive approach might be needed.
Soaking the end of the cylinder in Hydrogen Peroxide will dissolve or soften lead, but it might also damage the finish.
And use a bronze brush, instead of a mere toothbrush.
For the other crud, try a solvent with ammonia or acetone.
Some caution is required