You will run into a chambering problem with a cast bullet before you run into pressure issues. Even "hard" cast lead bullets are much softer than jacketed bullets and will swage down to bore diameter under even moderate powder pressure. If your bore slugs .308 and you aren't achieving good accuracy with a .309 sized bullet, chances are something else is amiss. What does the bore slug just ahead of the chamber? Is the muzzle crown scratched (even a small amount can make a difference). What powder and loads did you try? In my Krag, SR4759 gives good results. However, there are other good cast bullet powders out there (5744, 2400, 4198, etc). For best cast accuracy with minimal effort, strive for 1600-1800 fps muzzle velocity range. Chances are, your best accuracy will be found there.