Cast bullets are a whole 'nuther question. First, which mold are you using? It matters. Second, depending on your alloy, some molds will cast bigger, some molds will cast smaller. I run a Lyman 311041, which is roughly comparable to the Lee C309-170-F and it comes out at about .311. I lube it with alox, then run it thought the Lee .309 sizer to seat the gas check, then I tumble it in alox again. I can push those bullets to about 1850 fps in my Win94. My rifle has a 1:12 tube and 1850 is about as fast as I can push that bullet before it starts spinning itself apart. That same cartridge from a 1:10 Marlin barrel will show signs of failure, so those extra two inches mean something with my alloy.
Lots of guys use the Lee molds with good effect, so put yourself at ease about that. You can make bullets on a campfire, it's that simple. It's also complicated in that you're introducing other variables such as alloy into the equation. Ten years ago, I could tell you what a wheelweight was made of. Today, it's a crap shoot. Some seem to be pure lead, others are made of zinc. There is no way of telling without looking.
Still, it's an addictive hobby and I certainly don't want to dissuade you. Buy a copy of the Lyman Cast Bullet handbook, or a copy of Lee's Modern Reloading. Read them both and you should have the fundamentals on bullet casting.