I mentioned that earlier & pulled the post. I also posted up 2 links to help with determining how leading is being caused. It seems that the OP did not want to entertain the thought of his having a boolit that was large enough "as cast" to start with at .402 -.403, & then sizing it even smaller(.401) than his slugged bbl.(.4015) would likely cause leading, PC or lubed either way.
So, perhaps the OP will pay attention now that you mentioned it again.
Maybe the colored boolits in the other post & offer to get some, was distracting...
Here is that earlier deleted post if anyone cares now...:
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These links should be helpful, if you care to go look:
http://www.lasc.us/TaylorLeadingDefined.htm
http://www.lasc.us/Fryxell_Book_Chapter_7_Leading.htm
I would agree with boolits likely being undersized. If your bbl. slugged at .4015, then IMO, you should be trying to run your boolits at .4025-.403., meaning 0.001" over groove diameter(.4015)
With the undersized boolit, "gas cutting" becomes an issue & that will run the streaks up the bbl to the muzzle, as well as erode what lube or PC you have on the boolit.
Additionally... If it is a newer bbl. there could also be some stripping of your PC or lube by the rifling as well, particularly when the boolits are undersized.
Anyway, I hope ya figure things out & work out the issues.
G'Luck!