Since my right shoulder is kind of humped, I've been shooting my .40 cal Kuntz rifle a lot. Lately, we've been shooting around 40 shots on a Sunday, and I never have to clean until I take her home. Then, I get only grey water from flushing the barrel in a bucket of cold water, using a tight cotton flannel patch and a stainless cleaning rod.
I shoot a .395 pure lead ball, a .019" denim patch soaked with pure neatsfoot oil, 65 grains of 3Fg GOEX and load easily with a 5/16" hickory rod. If there is a secret, it may have to do with the alloy of your lead....must be pure lead, and with the lubrication of the patch. The lube must take ALL of the fouling from the last shot off of the bore, and push it down to the powder with the ball. So upon firing, all that remains is the fouling from one shot, no matter whether it is the second or fifty second shot. I confess to having a build up of fouling at the plug, which over the coarse of the day, causes the ball to seat a little higher than it did when I started. But it does not interfere in any way with the shooting, or the loading. I also use a short starter the seat the ball first flush with the muzzle, then 6" down the bore. It's the way I started and I see no reason to change now.
The humidity here is low in the winter. The lock and side of the barrel at the vent is always caked with white fouling at the end of the day. Tomorrow, the weather will likely be well above freezing...it's 48 F out there this afternoon.