I think some of the reasons why my Magma bottom pour pot works as well as it does are:
1. Forty pounds of lead is a lot compared to the eight or ten pounds you can get into a Lee. You can cast for two hours without stopping to add ingots.
2. The Magma pot, according to Magma's PR lady, has a temperature swing of ten degrees. That implies an accurate thermostat. These are Magma's contributions.
3. (My contributions.) I melt my wheelweights in a cast iron dutch oven over a "turkey fryer" propane heater. While in the dutch oven I carefully flux the melt, scraping the sides thoroughly, and skimming off the dross. I do this two or three times until the surface of the melt is bright and shiny, almost like a mirror. Only then do I cast the melt into my ingot molds. Thus the alloy that goes into the Magma is as clean as I know how to get it.
4. Pot temperature. Others say that the lowest temperature that will fully fill out the mold is the way to go. I don't quarrel with that. I've had some problems with bullet fill-out at temperatures under 700 degrees, so I give it an extra 50 degrees and don't look back. I have set mine at 750 degrees which seems to work for me. I have also cast at higher temperatures with good results, though the process is a bit slower because you spend more time waiting for the mold to cool down prior to dropping the bullets from it.
5. I've noticed that flow from the pot is diminished when it gets down to about a quarter full. I stop then, put back the sprues, and if I'm going to cast some more, add some ingots.
6. I get very few reject bullets. Part of that, I think, is because I use a consistent pattern to my casting. I don't try to set any records; between 200 and 250 bullets per hour, using a two cavity mold, satisfies me. I place the mold on the mold rest under the pot and fill the mold. Leave it there for a few seconds until the sprue has frozen. Pull it back and watch it for a few more seconds. At first, the sprue will be shiny. That fades into a rather attractive silver frost, which in turn fades into a very dull looking surface. At this point I cut the sprue and drop the bullets. If you get in a hurry and cut the sprue too soon you will get a lead smear on the underside of the sprue plate. You will also get a rough looking sprue area in the center of the bullet where the sprue cutter has torn the sprue from the bullet. Done properly you will have a slick looking sliced look on the base of the bullet and your bullets will have consistent weights.
6. I use Veral Smith's LBT molds. I haven't used a lot of other molds, a couple or three brands I guess. I don't need to try any others. My mind is closed on this subject.
7. Now for what some will call heresy: Smith advocates preheating the mold by dipping a corner into the melt. I have done this with every mold I have. Stick a corner of the cold mold into the melt and count off ten seconds. Take it out for another ten. Then repeat this cycle two more times. Most likely the first bullets you cast that session will be good. If they are not it is because you have a bit of oil in the mold. Wipe that out with a scrap of a paper towel, and go to it. No, my molds have not warped. If they did, I would quit this horrendous practice.