When you have your oiled fired furnace inspected for efficiency they check stack temperature, CO2, and your smoke number. These tests determine how efficiently you are burning your oil.
Your shooting a .222 at a rated chamber pressure of 46,000 cup or 50,000 psi and you will see more carbon flakes at lower chamber pressures. My Winchester 30-30 Trapper model with a 16 inch barrel does this all the time at 38,000 cup or 42,000 psi. It has to do with the peak flame temperature, burning efficiency and leaving combustion deposits inside the bore and and cartridge.
Quick load data for your load.
44,474 psi, 3066 fps, 94.91% of powder burnt
If this was your car and I was looking at your spark plugs I would say you need a hotter spark plug and your burning too rich. In simple terms you are not at 100% combustion efficiency and your leaving combustion byproducts in the case.
This happens all the time with reduced loads and low chamber pressures.