Basics for cast bullets in rifles:
1. Bullet diameter. Driving bands must match groove diameter. Nose area must match bore diameter. +.0001" is better than anything under actual diameter. Nominal diameters in .30 caliber are .300" bore, .308" groove diameter, although many production rifles will differ so slugging the bore is the prefered method for determining what is required (see further notes below on adjusting bullet alloys for different mold performance).
2. Bullet strength. Alloy used must be capable of standing up to the pressures involved. Very low velocity loads can use just about any lead alloy, higher velocities will require the stronger alloys.
3. Lubrication. Rifles are far more critical than handguns due to the longer trip down the bore. Nothing beats the NRA Alox formula, although white automotive lithium grease works very well also.
4. Gas checks. Copper gas check prevents bullet base from melting due to heat of expanding powder gases, smearing lead throughout bore. Also provides a scraping action to remove fouling.
5. Powder selection. Faster burning powders reach peak pressures more quickly, which can cause the bullet to slump, swell and deform (obturate) and affect accuracy in flight. Slower burning powders achieve peak pressures more slowly, however lower density loads can result in inconsistent ignition. Generally speaking, powders in the middle of the burning rate range will provide better overall performance more consistently. (NOTE: loading density can have a big effect on powder performance. Ideally, the powder charge should occupy nearly all of the cartridge interior space. When working with reduced loads a filler of loose Dacron fiber-fill, available from any fabric store, usually about 1 to 2 grains in weight, is useful for keeping the powder charge contained at the rear of the case for more consistent ignition.)
6. Bullet sizing. All sizing operations are damaging to cast bullets. If a bullet requires any significant sizing you are unlikely to achieve high accuracy in use. A bullet that drops from the mold at the exact diameter required, or about 0.001" larger, is preferable to any sized bullet. (NOTE: bullet alloy in use can have a big effect on actual bullet diameters, with the harder alloys like Linotype, etc, providing much less shrinkage as they cool. Experimenting with different alloys can make a big difference on the target grouping.)
7. Rifle cases used for reduced pressure loads should be segregated and never again used for full power loads. Reduced pressures can result in cases shortening, thus causing insufficient headspace. I usually load rifle cases to full power 3 or 4 times, then rotate those into my cast bullet stock. With cast bullets and moderate loads I have never worn out a cartridge case, with some having been loaded 30 times or more.
8. Bore must be completely cleared of all jacketed bullet fouling and residue prior to using cast bullets. This can have a huge effect on accuracy.
9. Seating cast bullets in the cartridge cases can result in damaging the bullets. Use of the Lyman "M" die for neck flaring makes bullet seating easy and avoids this problem completely.
I have used cast bullets in .30-30, .30-40 Krag, .300 Savage, .308 Winchester, and .30-06 for years. Favorite molds are the Lee C309-180R, Lee C309-170RF, cast of 50/50 mix Linotype and wheelweights, with Hornady crimp-on gas checks, Alox lubricant applied in the .309" lubri-sizer die. Moderate charges of powders such as H335, BLC-2, and 4895 will provide excellent results in all these calibers at velocities in the 1800 to 2100 FPS range (very comparable to factory .30-30 ammunition). My sons and I have taken many mule deer with these loads at ranges to 150 yards, seldom requiring more than one shot, and recovered bullets have shown performance similar to jacketed soft points as far as penetration, retained weight, and expansion are concerned.
I have also used cast bullets in practice ammunition for Civilian Marksmanship Program rifle practice, especially in the .30-40 Krag and .30-06 Springfields. In both I have found that sight settings can be easily adjusted, using the 300 yard sight setting for 100 yards and 500 yard setting for 200 yards, so I don't have to change my basic sight settings (for ball ammo) for my practice shooting. Groups from 3" to 4" at 100 yards are not difficult to achieve with open sight rifles and carefully adjusted cast bullet loads, plenty accurate for most hunting applications.
The Lee aluminum bullet molds are produced by the "hobbing" process, with the bullet cavity machined to near final dimensions, then a hardened core inserted and mold cavities completed by compressing the mold around the hardened core under high pressures, for a very consistent result. Most molds are produced by machining, so are more subject to variations due to tool wear. Aluminum molds are also very easy to use since lead will not adhere to aluminum, and the blocks can be preheated by dipping a corner into the melted bullet alloy. Lee molds are remarkably inexpensive as well.
Lots of fun through the years with cast bullets. Very involving hobby in its own right.
Enjoy.