I recently purchased a secondhand copy of Jacketed Performance with Cast Bullets by Veral Smith, 3rd edition. I promptly dug into it and am only a few pages shy of it's finish. There is some very interesting things in there.
One of which is that cast bullets seated with the first driving band below the neck of a bottle necked cartridge might be subject to base slump or obturation in the case.
Another is that gas checks need a space between their top and the bottom of the next driving band for lead shavings to collect. And that gas checked bullets lower pressure due to controlling obturation, thus reducing friction. I knew that gas checks reduced pressure, but I hadn't figured out why.
Friction is a product of both obturation and lube function, and when the lube reaches its limit friction increases because the lube stops coating the bullet body, and thus increases pressure and this is where leading occurs (at the velocity/pressure limit).
A little tidbit that will help me, filing a vertical vent line between the mould cavities to help venting of the second bullet after you've filled the first cavity. Often I've fought pour fillout in that second cavity and while I haven't had a chance to try it yet, I expect this to alleviate that nuisance.
Many of the things I learned, were things I knew, or suspected, from my own experiences but it was explained as to the why.
Overall I think it is a very worthwhile book to read and the knowledge contained therein will improve anyone's bullet making.
Bazoo