LODGEWOOD
The void you're seeing can be caused by poor casting technique. It's safe to shoot, but don't expect the best accuracy. The void is most often caused by a cold core pin and/or lead almost too cold.
Casting minies is fine with a bottom pour but the lead should be pure and run the pot and mold hot. The key to getting a good cast is to slightly tilt the mold as you start the lead pour and have the core pin hot. I cast at 850f, nearly as hot as my pot will go. BTW, I do have a 575213OS mold, but it's not for sale, trade, barter, beg, nada no way. Again, the guy at Lodgewood knows how to cast quality minies, he's NSSA too and not a round ball guy with a lead pot and a mold. I've never gotten one like that from him.
Sight picture. Some folks like 6oclock. I prefer center hold. For your purposes, either get a roll of brown paper that painters use for masking (available at your local big box hardware store) or just get a big cardboard box and use it flat. Use whatever sight picture you want but be consistent. You're looking for a group, not where the group is on the paper. Sight alignment is another subject for another time.