castboolits.com is a great reference for cast bullet ("boolit") shooters.
Many variables for accuracy in a rifle. Rifling twist, groove diameter, throat diameter, bullet hardness, powder, velocity etc.
I would recommend getting a good assessment of your barrel first. Do a "pound cast" (look it up) technique to accurately measure the THROAT diameter of your rifle/chamber ( 0.000" or 0.0000")
Then lube your bore, and tap the bullet you wish to use through your breach, out to the muzzle. Record GROOVE diameter (0.000", or 0.0000").
Use a tight fitting patch or bronze bore brush, and measure your rifling twist. (1-10"? 1-11"? 1-12"? other?)
A very good starting point for accuracy is to size the cast bullet to THROAT DIAMETER (maybe 0.0005" smaller than throat diameter)- or just small enough diameter to just fit the throat of the chamber. If too big, it won't fit. Too small, it will FIT, but won't give you best accuracy.
You will probably read that you should size your cast bullets to 0.001-0.002" over groove diameter. This is a good start, but is not really what you should target your bullet sizing diameter. This APPROACHES throat diameter, and may even match it (it the diameter matches it, you got lucky). You really want to target fitting just so it snugly fits into the throat. This will save you time in seaching for best accuracy with cast bullets.
watch your MV. Use gas checks. If your twist is 1-10" , you will likely see best accuracy around 1600 fps. If 1-12", you may be able to get good accuracy up to 1800-1900 fps, pending bullet alloy. If you are looking for "high velocity", you should seek even a slower twist of 1-13" or 14" twist, this may get you up to 2400+ fps with accuracy. The issue you run into is "threshold velocity" or "threshold rpm" (look that term up for cast bullets).
castboolits.com has a lot of this info on the website. One person's posts who you should read over there is "Larry Gibson". He has done a tremendous amount of work with high velocity cast bullets and slower rifling, due to the threshold rpm / velocity issue. It is essentially when the softer cast bullet strips from the rifling as it is pushed too fast for the barrel's rifling.
The SAECO 315 ("Eagan bullet") is a good bullet design, as is the 311041. "Ranch Dog" designed molds are excellent hunting bullets, and accurate, and have a large meplat ( flat nose for good energy transfer). Night Owl Enterprises (NOE) make excellent molds of proper diameter. Arsenal Molds is another excellent mold maker, and they will cut the mold to your requested diameter for no extra charge.
Good luck. Throat diameter is the key measurement.
