Been using the buffalo .395 425gr heavy PP bullet in the 40-65 rolling block. The barrel is a green mountain .40 cal they used to make, I do not remember off hand which profile it is, or the twist rate ( I do have the info written down somewhere, though not in front of me at the moment) it is a fairly heavy barrel with a nice taper which suits the action well. Rifle is very accurate out to 400 yards, but at 600, the group opens up multi directionally. Suggestions on a better bullet would be welcome.