From what I have read, 1/8" seems to be the recommended depth. I have gone deeper, but not having done much experimenting, I really don't know if it helps. If one goes too deep, the integrity of the bullet will most certainly be compromised. It could even get dangerous as it may let part of the bullet in the barrel if the charge shoots through the bullet. Kind of like the hollow base bullets. Don't really know. Still they are labor intensive and very annoying to clean the lead from the drill bit. Good-luck...BCB