Thanks Steve et al,

I also separate bullets from different sessions and I usually try to use bullets with no more than 1 gn of difference in weight.
In this case, a difference of 1 gn means 0.21%; 2 gn 0 .42%; 3 gn 0 .65% and 4gn only 0.87%.
IIRC most of bullet makers have more than 0.5% of variation in weight.
I will have to work on my casting and I will try some tin on my pot.
Thanks for all the advises.
Martin