I should have mentioned I am using 58 caliber molds to make slugs and the slugs are 600gr a piece so they are very large molds and I felt I needed the little extra volume to help with casting.
A couple weeks ago I was using a tiny Lee pot to cast 69 calibre slugs. Things went very well for me... no obstructions at all despite all the gunk I skimmed off.
http://www.canadiangunnutz.com/forum/showthread.php/1078355-Casting-69-calibre-lead-slugs