Wipeout is your freind. CR10 is dangerous. Leave CR10 in real long, and you might pull out surface rust. Been there.
Wipe out, you can leave in for days, and no harm. ON a badly fouled gun leave wipeout in for 5 hours, then clean out and apply again and let sit overnight. Then in the morning, swab all out and do it again. This time you should see no fouling.
If you do, repeat. You might want to use some aggressive JB bore paste.
Also, don't use a brass jag. This will react and give you false readings. I use the coated ones.