Ok, so I realized that during the casting process using a Master Caster lead somehow gets between the sprue plate and the upper part of the mold raising a bit the sprue plate. I've discovered that due to the white wall behind the machine and my position on the chair while casting.
When I started the sprue plate was in perfect contact with the mold but after aprox 1000 bullets I've seen that between the sprue plate and the mold there is a tiny gap and I could see the white wall behind the machine through the gap.
Took the mold out and realized there is something not right there.
First of all, how that could happen.
I unscrewed the sprue plate and I've seen lead spreaded on the path of the sprue plate while it moves and cut the bullets casting cone from the base of the bullets.
Another question is how can I remove the lead without damaging the mold. I tried with a "lead remover" solution but is not efficient enough. IT only works on mild lead fouling inside the barrel. I do have here serious smudges with lead.
As a note: I tighten the screw quite well so it is not because the sprue plate was loose.
The only explanation I can come up with is that some times I do not let the alloy to cool enough in the mold and cut the sprue while the lead is still soft. I can't think of anything else as a cause of this lead smudging between the sprue plate and the upper part of the mold.
Just watch the pictures below.
Thank you