Does that sound right as it's below the melting point of lead.
Yes. That is why one must be careful to not get their pot too hot when casting with tin/pewter, or it will oxidize to the surface faster than the lead and the caster will skim it off as scum/dross and lose his precious metal. It must be fluxed back into the melt occasionally, rather than skimmed off. So if you are casting bullets with tin/pewter in the mix, try not to get the alloy over 700° for any length of time and you will cast it faster than you are losing any appreciable amount to oxidation.