The porous looking surface. If you are using a bottom pour pot and the wait between casts is long enough, the drop of lead hanging in the spout can oxidize and this drop will be the first to hit the bottom of the mold, normally on one side, because few of us hold the mold perfectly level. To see if this is your problem, put some sort of shallow metal cup under the spout so you can trigger a little spurt and have it be caught in the cup. The cup needs to be shallow enough to let you hold the mold over it of coarse. I've cast a lot this way, to keep bullets pretty, yet, the little bit of oxidization doesn't hurt anything except cosmetics. If you try this suggestion and it doesn't fix the problem, let me know. Off hand, I can't think of any other reason for the problem.
To get better fillout near the bottom of the bullet, pour a fast stream, and cap the sprue plate as high as possible, to put a little pressure on the lead. This will normally fix the problem you are having, and also slightly rounded bases.
Whenever one believes he might be getting porosity in his bullets, a sprue that remains molten till the bullet has a chance to solidify completely will cure the problem. You'll notice that my sprue plates hold larger sprues than any other manufacture, and also that the sprue plate metal covering the bullet base is very thin. These two features are not accident but were carefully designed to solve the bullet porosity problem. The thin cross section over the bullet is quickly heated to above the melt temp, and being steel (relitively low heat transfer rate, compared to the aluminum blocks) and very thin cross section, with high thermal mass in the huge sprue, heat is retained here longer than any other manfactures molds.
If you understand what I desingned into my molds and work with it, there will be NO weight variation. I and thousands of customers are very proud of that little fact, though few understand why LBT molds control porosity so well.
