The thing about .30 cast bullets is you are limited to .30-30 performance or a bit better before accuracy heads south and the barrel starts to lead- given bullet alloys that are conducive to expanding on game. There are certainly many exceptions to that rule, but let it suffice for a raw beginner. (Composite bullets, paper patching, etc.)
That is why the .30-30 and similar cartridges are so versatile- you can get factory performance with lead bullets at a tiny fraction of the cost.
To make up for that shortcoming, what one does in larger capacity cases is run heavy bullets. For example, I shoot a 220 grain cast bullet in .30-06 at 2000fps, which puts it squarely in the power range of the .30-40 Krag which was (and is) considered a darn fine big game cartridge. (A trick that is certainly repeatable in a .308, I would presume.)
The only other way to beat the shortcomings of cast bullets is to go to a larger diameter cartridge, say .35 or .45 caliber, where you have bullet weights available that when driven at .30-30 velocities (2000fps give or take) provide some truly impressive energy figures.
That said, a 180-190 grain .30 cast bullet, either round nose or flat nosed, driven at initial velocity of 2000fps, is a darned fine killer of deer sized game. I've been doing it for years, and have had a ball with experimenting also, not to mention probably saved enough in ammo costs to buy a couple nice rifles.
