Forming 300 AAC Blackout Cases

Should you choose to make 300 AAC Blackout cases from 223 Remington or 5.56 cases, you may have to ream the case necks to obtain a maximum loaded case neck outside diameter of .3340". The reason for this is that the case wall thickness of a rifle case is tapered, being thinnest at the case neck and thicker towards the case head. When you trim the longer case to the shorter dimension, you are getting into a thicker case wall. The maximum allowable case neck wall thickness is .013" when seating a .308" bullet. Because of the taper of the case, wall thickness at the case neck mouth may be .013" but could be thicker just above the shoulder. The thicker wall in this area will cause the loaded case neck outside diameter to be in excess of .3340". This will cause the bullet to be pinched by the case when the round is chambered, which may result in dangerously high chamber pressures.