This lead thing can be a puzzling at times. At times using the same loading components and procedures, you don't have a problem then you do
There a few things I found that causes leading in my rifles sometimes.
By the way there is some lead in every rifle if you see it or not, it might be hidden under the black fouling and comes out with the first patch.
This is what I found, and I did this on purpose to see if that's the reason for the lead in my barrel.
Short cases I found and that is easy to check I found causes it.
Take a .45-2.1 case full length size it then trim it to 2.980 like some do that case can be as short as 1.95" depending how liberal your chamber is when the case walls get pushed back out fitting the chamber.
Well there will be a gap between the case mouth and the chamber when that case expands and the bullet will shave some lead when it obtrudes and gets swaged back down to were it fits the bore again, and if the wad does not carry it through as it passes and the hot soft lead stays behind and gets ironed in with the next shot.
I have no way of proving my thought, just a guess.
By not having good case prep while loading might give a problem.
By not having the case mouth chamfered and properly expanded while you seat the bullet you will shave lead off the side of the bullet and it will get trapped by the lube on the bullet side wall and get ironed on as the bullet passes through the bore.
Dry fouling will lead the bore.
To hard and small bullet diameter and improper wad diameter were the hot gas will cut the bullet side wall will lead the bore.
I have loaded PP and GG bullets not using any wads and they shot good with out any lead problems.
This is just my thought on this.
Kurt