I haven't heard anyone mention this yet, so I'll tell you what I do when I need to fill out the mold better.
I also cast bullets (or boolits as us casters call them) and I am far more picky about filling out the entire mold cavity when casting my lead bullets (need to have consistant weight, lube ring grooves, OD, .etc) I could get into more detail but this is the lure making forum.
I used to buy wheel weights and melt those down, flux, and re-flux but my lead was very inconsistant. I now purchase lead from a local indoor gun range that comes out of the bullet traps. Bullet lead is very soft compared to wheel weight lead. This lead is soft because it's typically held together with a FMJ (copper) or copper plating. Afterall, you want bullets to expand, so the softer the lead, the better (as long as it jacketed).
My "go-to" solution for filling out a mold when pouring, is the addition of Tin. I buy the thick plumbers lead free solder from Home depot that is typically used for brazing or soldering copper pipe. The key is to not add too much. I usually aim for 1-2% of my lead to be Tin. It really helps fill the mold out.
No Tin on the left and Tin added to the right
Solder I use
I also have some molds that don't like to drop my casting out. For this reason, I keep a 4x4 block of wood laying on top of my casting bench and i simply give the mold a firm, but no too hard, whack on the wood block and my castings fall right out.
Just for reference, I do use a Lee 5lb bottom pour pot for all of my castings and I flux my lead (once per full pot) with cheap dollar store birthday candles. I will admit, my very first time was quite amusing and I was not expecting the pot to catch fire while the wax burned off.
I hope my little tips and tricks can help you out (or anyone for that matter) as well if you choose to try them.