My Kimber is a 223 (Longmaster Classic) witha 1:9 twist 24" barrel. I tried every trick in the book and several different primers, powders, and bullet manufacturers to get it to shoot 55 grain bullets without success. I thought there was something wrong with the gun until I tried a 40 grain Nosler BT, then I couldn't get a bad group no matter what I did. I found the Sierra 63 grain semi point bullets shoot really well too but bullets that are >40 grains, <63 grains no luck at all. Another bullet that shot very accurately in my gun is the Speer 70 grain round nose. My chamber requires it be deeply seated and the powder is compressed but it is an accurate bullet in my gun.
