Cast bullet accuracy comes from properly matched bullet diameter to your gun’s dimensions of throat, and groove diameter.
Slugging your bore to get groove diameter, and then adding .001-.003” more for bullet diameter is a good start, but does not address the key measurement: THROAT diameter.
You need to find throat diameter. This is the variable we try to address by adding bullet diameter excess to groove diameter. The trouble is, the “ extra amount” over groove diameter is only a guess. You need to take that variable out, and get an accurate measurement of your throat. Then select a sized bullet to just fit into the throat with a loaded round ( about 0.0005” less than throat diameter).
Take the guess work out. Save time, money, frustration with molds, sizing does, etc. Do a pound cast of your throat, and use that data for an informed decision on bullet diameter/ mold selection.
Veral Smith of LBT molds has info on his website for it.
Here is another link with a write up about making a “ pound cast.” If you are a cast bullet shooter, you should fully understand this concept. It take away most of the mystery of poor cast bullet performance.
https://www.artfulbullet.com/index.php?threads/article-2-how-i-make-pound-casts.131/
