As the former owner of Meister Bullets, inc. (the oldest and largest manufacturer of cast bullets in the U.S.) I can answer your question......we found at Meister that, for accuracy and to prevent leading (a real concern when shooting lead alloy bullets, especially if you are shooting "cheaply made" lead alloy bullets....yes, there IS a difference), we recommend shooting a hard cast, lead alloy bullet either .001" or .002" LARGER than barrel diameter. This will provide you with the best overall performance from lead alloy bullets. However, not all barrels are the same. Firearms mfgs advised us that two guns can come down the assembly line side by side and will have different diameter barrels due to cutting die wear. In reality, no two barrels are the same. So the trick is to know what your barrel diameter is and THEN size a bullet .001" to .002" larger than that diameter. The new owners of Meister Bullets have discontinued it's "Slug-Your-Barrel" kits but still have a few listed on their website. Check them out and if they have a kit for the caliber you are wanting to load buy the kit and use it to measure your gun's barrel diameter. It's quick and, most of all, very accurate, as well as being simple to use. The kit comes with everything you need to accurately measure your gun's barrel diameter. Then, match the bullet to your gun's barrel diameter. Have fun and pay attention to your reloading!!