Cris,
FIRST! Do not mix that alloy with any others that you may have,until you are sure that it is safe.
Then you cast some bullets with the alloy,and shoot them with a known,reliable charge.
If the alloy contains some contaminent,like Zinc,the grooves on the bullet will not ever fill out. If you have 15% tin,or more,You will have some left in the barrel.
Frank