The last two casting sessions I've done I've been having a terrible time with air pockets or voids inside the base of the bullet. Now, I don't know if they're gas bubbles or just voids, but when I drop the bullet the side of the base shrinks and sort of caves in on itself. Sometimes you can actually watch it happen. It looks like someone whacked the side of the bullet with a tiny hammer and dented it. I culled all the dented ones, which was about half of them, and shot the normal looking ones (that appeared to be completely filled out) only to find that I'm getting fliers that are all over the place (some were 12 inches away from the main group at 100yd). I collected some of the shot bullets and found the sides of the bases caved in, so there were voids in a lot of those too even though they looked fine. Last time I cast a large batch with this mold I had fill out problems with the base, but culling the bad looking ones left me with good shooting bullets.
Any ideas on what can cause these voids? Something in the melt?
I tried different casting methods, I tried pressure casting and pouring a generous sprue and neither method solved it. I'm casting at 650 degrees and using straight clip on WW lead that I smelted myself in a separate pot. The mold is a Lee 230gr "blackout" mold which I've previously gone over to make sure the vent lines are open, I'm lubing the sprue plate with a q-tip using a very light coat of two stroke synthetic oil.
I'm considering just giving up on this mold and buying an aluminum NOE 247gr mold but I don't know if it will give me the same issues.