Ok, I think y'all have convinced me lol. With that said, at how many fps do you start to see them leading the barrel?
It can start at about as low as 300, and go from there.
Leading in bores has a multitude of nefarious origins :
Incorrect slug size.
Too hot of a powder charge.
Incorrect hardness for application.
Forcing cone irregularities.
Improper bullet seating techniques.
Pre-existing bore obstructions.
Pre-existing bore irregularities.
Improper bullet lubrication amount.
Improper bullet lubrication type.
Nearly limitless Combinations of all of the above.
In the application it would appear you intend to use them( targets), a bullet of anywhere between 16 and 18 BHN , driven under 1400 fps, given proper bullet fitment to both bore and forcing cone, should produce minimal to no leading.
In my .44 magnum, a 240g tumble lube bullet of 16-18 BHN on top of 10 grs of unique is excitingly accurate, and produces little to no leading.
That same bullet on top of 23 grs of h-110 produces noticeable leading for the first 1 inch or so of the bore. With that said, I can shoot 4-5 cylinders of this load in succession, at ranges of 15-30 feet, before any appreciable loss in accuracy occurs. At that point, its probably my hand- not the leading.
In any event, 2 or 3 jacketed or gas-checked rounds after each lead firing session removes the lead from my bore, and that of many others that I have tried, completely.
Why I am awake at this time to answer this question however, is another matter entirely.