I normally clean my barrels with Hoppes Benchrest solvent , then dry the bore and oil with Transmission fluid because it leaves a thinner coating of oil that is easy to remove. If I forget to remove the oil with a clean patch the first few shots will leave what looks like heavy powder fouling but after five or six shots things get back to normal. H4831 will leave a bit of fouling with low pressure loads.

M1Tanker is right on with the CLP issue I think.

I was in the Navy and our GM's swabbed the bores of the deck guns with it. Although there was evidence it's a good rust preventer I swab out the stuff before shooting simply based on what the big gun looked like after only firing a few rounds at a gun boat. It was dirtier than I could have imagined.

If you don't have rusting problems I'd not put it in the bore at all unless it's going into storage.

The other thing as mentioned is the ammonia salts being left behind from a copper removal session. The salts tend to stay behind and can be a little stubborn to remove. I'd high recommend swabbing a bore with solvent and a light oil after using a copper remover. I use Kroil as a final wipe after cleaning unless I'm expecting to go shooting again soon then I'll leave it basically dry. The Kroil is thin and passing a dry patch through after oiling is enough to get the excess but may leave a thin layer on the steel. Haven't had any problems doing it this way yet. Nate

Took my rifle to the range last night just to check zero from last season. Spent lots of time last year working up a load but have not shot this rifle since last deer season. Only took 5 shots to prove it is still right where it should be and shooting good. I knew I pulled two shots but the 3 good ones clover leafed at 100 yards. However when I got home to clean it I noticed a bunch of fouling in the middle of the bore. This fouling really took some soaking and brushing to get out. It almost looked like the bore had rusted and pitted it looked so bad but it was indeed powder fouling. I was shooting just as dusk was approaching and it did cool off and was very humid. my target actually was damp when I pulled it. Could this have caused that? In 12-13 years I never remember this rifle getting this way.
The load is 73 grains of H4831SC under 180 grain Nosler Ballistic Tip.

Was it cleaned the last time you shot? Five rounds can foul a barrel, especially if you have a rough spot or two however. How about copper?