The easiest way to clean lead deposits off that area is to shoot a couple of cylinders of hot jacketed rounds. Powder residue isn't much of a problem. Lead buildup is. One thing to check is endshake. If the cylinder can move front to back, it makes any leading problem worse. While you're at it, check the timing and barrel to cylinder alignment. If these are off, it can contribute to lead shaving. Additionally, you can spray Dillon case lube on the forcing cone and front of cylinder every 50 shots or so. This will make any deposits not build up as much and easier to clean off what does build up.